diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 6d94e410929..d3077dc485e 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -2,8 +2,9 @@ includes:
     - phpstan-baseline.neon
 
 parameters:
-    level: 3
+    level: 4
     paths:
         - src
     excludePaths:
         - src/Test
+    treatPhpDocTypesAsCertain: false
diff --git a/src/Environment.php b/src/Environment.php
index 80c578d766d..a9cbb0b44e5 100644
--- a/src/Environment.php
+++ b/src/Environment.php
@@ -371,6 +371,7 @@ public function loadTemplate(string $cls, string $name, ?int $index = null): Tem
 
             if (!$this->isAutoReload() || $this->isTemplateFresh($name, $this->cache->getTimestamp($key))) {
                 $this->cache->load($key);
+                /** @var class-string $cls -- to reset `class_exists($cls, false)` result for PHPStan */
             }
 
             if (!class_exists($cls, false)) {
@@ -379,6 +380,7 @@ public function loadTemplate(string $cls, string $name, ?int $index = null): Tem
                 if (!isset($this->hotCache[$name])) {
                     $this->cache->write($key, $content);
                     $this->cache->load($key);
+                    /** @var class-string $cls -- to reset `class_exists($cls, false)` result for PHPStan */
                 }
 
                 if (!class_exists($mainCls, false)) {
diff --git a/src/Error/Error.php b/src/Error/Error.php
index 3ccdda2cdc3..bfb5f6e44dd 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -142,12 +142,7 @@ private function updateRepr(): void
         }
 
         if ($this->name) {
-            if (\is_string($this->name) || $this->name instanceof \Stringable) {
-                $name = \sprintf('"%s"', $this->name);
-            } else {
-                $name = json_encode($this->name);
-            }
-            $this->message .= \sprintf(' in %s', $name);
+            $this->message .= \sprintf(' in "%s"', $this->name);
         }
 
         if ($this->lineno && $this->lineno >= 0) {
diff --git a/src/Extension/CoreExtension.php b/src/Extension/CoreExtension.php
index 88dbc58a46c..9f01341dd3b 100644
--- a/src/Extension/CoreExtension.php
+++ b/src/Extension/CoreExtension.php
@@ -1199,7 +1199,7 @@ public static function length(string $charset, $thing): int
             return mb_strlen($thing, $charset);
         }
 
-        if (is_countable($thing) || $thing instanceof \SimpleXMLElement) {
+        if (is_countable($thing)) {
             return \count($thing);
         }
 
diff --git a/src/NodeVisitor/EscaperNodeVisitor.php b/src/NodeVisitor/EscaperNodeVisitor.php
index 31e0bd75e19..851bde43bf8 100644
--- a/src/NodeVisitor/EscaperNodeVisitor.php
+++ b/src/NodeVisitor/EscaperNodeVisitor.php
@@ -71,7 +71,7 @@ public function enterNode(Node $node, Environment $env): Node
         return $node;
     }
 
-    public function leaveNode(Node $node, Environment $env): ?Node
+    public function leaveNode(Node $node, Environment $env): Node
     {
         if ($node instanceof ModuleNode) {
             $this->defaultStrategy = false;
diff --git a/src/NodeVisitor/OptimizerNodeVisitor.php b/src/NodeVisitor/OptimizerNodeVisitor.php
index 4f92ce14f14..734363f0e8e 100644
--- a/src/NodeVisitor/OptimizerNodeVisitor.php
+++ b/src/NodeVisitor/OptimizerNodeVisitor.php
@@ -72,7 +72,7 @@ public function enterNode(Node $node, Environment $env): Node
         return $node;
     }
 
-    public function leaveNode(Node $node, Environment $env): ?Node
+    public function leaveNode(Node $node, Environment $env): Node
     {
         if (self::OPTIMIZE_FOR === (self::OPTIMIZE_FOR & $this->optimizers)) {
             $this->leaveOptimizeFor($node);
diff --git a/src/NodeVisitor/SafeAnalysisNodeVisitor.php b/src/NodeVisitor/SafeAnalysisNodeVisitor.php
index a38bb63ce49..1dce9b51b5e 100644
--- a/src/NodeVisitor/SafeAnalysisNodeVisitor.php
+++ b/src/NodeVisitor/SafeAnalysisNodeVisitor.php
@@ -84,7 +84,7 @@ public function enterNode(Node $node, Environment $env): Node
         return $node;
     }
 
-    public function leaveNode(Node $node, Environment $env): ?Node
+    public function leaveNode(Node $node, Environment $env): Node
     {
         if ($node instanceof ConstantExpression) {
             // constants are marked safe for all
diff --git a/src/NodeVisitor/SandboxNodeVisitor.php b/src/NodeVisitor/SandboxNodeVisitor.php
index f9724353d1f..1cb4bcb16a4 100644
--- a/src/NodeVisitor/SandboxNodeVisitor.php
+++ b/src/NodeVisitor/SandboxNodeVisitor.php
@@ -103,7 +103,7 @@ public function enterNode(Node $node, Environment $env): Node
         return $node;
     }
 
-    public function leaveNode(Node $node, Environment $env): ?Node
+    public function leaveNode(Node $node, Environment $env): Node
     {
         if ($node instanceof ModuleNode) {
             $this->inAModule = false;
diff --git a/src/Parser.php b/src/Parser.php
index 1b10a656187..6fdb9cbd5bc 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -152,6 +152,9 @@ public function subparseIgnoreUnknownTwigCallables($test, bool $dropNeedle = fal
         }
     }
 
+    /**
+     * @phpstan-impure
+     */
     public function subparse($test, bool $dropNeedle = false): Node
     {
         $lineno = $this->getCurrentToken()->getLine();
@@ -362,7 +365,7 @@ private function filterBodyNodes(Node $node, bool $nested = false): ?Node
         // we need to discard the wrapping "Node" for the "body" node
         $nested = $nested || !$node instanceof Nodes;
         foreach ($node as $k => $n) {
-            if (null !== $n && null === $this->filterBodyNodes($n, $nested)) {
+            if (null === $this->filterBodyNodes($n, $nested)) {
                 $node->removeNode($k);
             }
         }
diff --git a/src/Profiler/NodeVisitor/ProfilerNodeVisitor.php b/src/Profiler/NodeVisitor/ProfilerNodeVisitor.php
index ad7ce623400..a587260dd70 100644
--- a/src/Profiler/NodeVisitor/ProfilerNodeVisitor.php
+++ b/src/Profiler/NodeVisitor/ProfilerNodeVisitor.php
@@ -41,7 +41,7 @@ public function enterNode(Node $node, Environment $env): Node
         return $node;
     }
 
-    public function leaveNode(Node $node, Environment $env): ?Node
+    public function leaveNode(Node $node, Environment $env): Node
     {
         if ($node instanceof ModuleNode) {
             $node->setNode('display_start', new Nodes([new EnterProfileNode($this->extensionName, Profile::TEMPLATE, $node->getTemplateName(), $this->varName), $node->getNode('display_start')]));
diff --git a/src/TokenStream.php b/src/TokenStream.php
index 4ff299d5ed0..191c91085ca 100644
--- a/src/TokenStream.php
+++ b/src/TokenStream.php
@@ -44,6 +44,8 @@ public function injectTokens(array $tokens)
 
     /**
      * Sets the pointer to the next token and returns the old one.
+     *
+     * @phpstan-impure
      */
     public function next(): Token
     {
@@ -58,6 +60,8 @@ public function next(): Token
      * Tests a token, sets the pointer to the next one and returns it or throws a syntax error.
      *
      * @return Token|null The next token if the condition is true, null otherwise
+     *
+     * @phpstan-impure
      */
     public function nextIf($primary, $secondary = null)
     {
@@ -66,6 +70,8 @@ public function nextIf($primary, $secondary = null)
 
     /**
      * Tests a token and returns it or throws a syntax error.
+     *
+     * @phpstan-impure
      */
     public function expect($type, $value = null, ?string $message = null): Token
     {
diff --git a/src/Util/CallableArgumentsExtractor.php b/src/Util/CallableArgumentsExtractor.php
index 8ca8bb602c5..c129a320a0d 100644
--- a/src/Util/CallableArgumentsExtractor.php
+++ b/src/Util/CallableArgumentsExtractor.php
@@ -39,6 +39,7 @@ public function __construct(
      */
     public function extractArguments(Node $arguments): array
     {
+        /** @var array<int|string, Node> $extractedArguments */
         $extractedArguments = [];
         $extractedArgumentNameMap = [];
         $named = false;
diff --git a/src/Util/ReflectionCallable.php b/src/Util/ReflectionCallable.php
index 5fd5d1ba754..31207201bd0 100644
--- a/src/Util/ReflectionCallable.php
+++ b/src/Util/ReflectionCallable.php
@@ -77,7 +77,7 @@ public function getReflector(): \ReflectionFunctionAbstract
         return $this->reflector;
     }
 
-    public function getCallable(): \Closure|string|array|null
+    public function getCallable(): \Closure|string|array
     {
         return $this->callable;
     }