diff --git a/Rethinking_2/Chp_04.ipynb b/Rethinking_2/Chp_04.ipynb
index 436db3a..6b882f2 100644
--- a/Rethinking_2/Chp_04.ipynb
+++ b/Rethinking_2/Chp_04.ipynb
@@ -1112,7 +1112,12 @@
    ],
    "source": [
     "trace_df = pm.trace_to_dataframe(trace_4_1)\n",
-    "trace_df.cov()"
+    "trace_df.cov()\n",
+    "\n",
+    "# update: trace saved with InferenceData format\n",
+    "# trace_to_dataframe() retained until update to InferenceData is stable\n",
+    "with pm.Model() as m4_1:\n",
+    "    pm_data = az.from_pymc3(trace=trace_4_1)"
    ]
   },
   {