Bump Phpstan to level 4 #3114

Job	Run time
PHP 8.2	19s
PHPStan (8.4)	16s
PHP 8.3	10s
PHP 8.4	15s
Integration tests with PHP ${{ matrix.php-version }}	0s
cache-extra PHP 8.2	10s
cssinliner-extra PHP 8.2	10s
html-extra PHP 8.2	12s
inky-extra PHP 8.2	9s
intl-extra PHP 8.2	13s
markdown-extra PHP 8.2	10s
string-extra PHP 8.2	13s
twig-extra-bundle PHP 8.2	12s
cache-extra PHP 8.3	10s
cssinliner-extra PHP 8.3	9s
html-extra PHP 8.3	8s
inky-extra PHP 8.3	10s
intl-extra PHP 8.3	9s
markdown-extra PHP 8.3	18s
string-extra PHP 8.3	9s
twig-extra-bundle PHP 8.3	11s
cache-extra PHP 8.4	14s
cssinliner-extra PHP 8.4	9s
html-extra PHP 8.4	12s
inky-extra PHP 8.4	15s
intl-extra PHP 8.4	15s
markdown-extra PHP 8.4	10s
string-extra PHP 8.4	13s
twig-extra-bundle PHP 8.4	10s
	5m 31s

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