diff --git a/notebooks/unit4/unit4.ipynb b/notebooks/unit4/unit4.ipynb
index 45d1c0c..bfc44c5 100644
--- a/notebooks/unit4/unit4.ipynb
+++ b/notebooks/unit4/unit4.ipynb
@@ -7,7 +7,7 @@
         "colab_type": "text"
       },
       "source": [
-        "<a href=\"https://colab.research.google.com/github/huggingface/deep-rl-class/blob/ThomasSimonini%2FPG/notebooks/unit4/unit4.ipynb\" target=\"_parent\"><img src=\"https://colab.research.google.com/assets/colab-badge.svg\" alt=\"Open In Colab\"/></a>"
+        "<a href=\"https://colab.research.google.com/github/huggingface/deep-rl-class/blob/GymnasiumUpdate%2FUnit4/notebooks/unit4/unit4.ipynb\" target=\"_parent\"><img src=\"https://colab.research.google.com/assets/colab-badge.svg\" alt=\"Open In Colab\"/></a>"
       ]
     },
     {
@@ -229,6 +229,15 @@
         "- `gym-games`: Extra gym environments made with PyGame.\n",
         "- `huggingface_hub`: 🤗 works as a central place where anyone can share and explore models and datasets. It has versioning, metrics, visualizations, and other features that will allow you to easily collaborate with others.\n",
         "\n",
+        "You may be wondering why we install gym and not gymnasium, a more recent version of gym? **Because the gym-games we are using are not updated yet with gymnasium**. \n",
+        "\n",
+        "The differences you'll encounter here:\n",
+        "- In `gym` we don't have `terminated` and `truncated` but only `done`.\n",
+        "- In `gym` using `env.step()` returns `state, reward, done, info`\n",
+        "\n",
+        "You can learn more about the differences between Gym and Gymnasium here 👉 https://gymnasium.farama.org/content/migration-guide/\n",
+        "\n",
+        "\n",
         "You can see here all the Reinforce models available 👉 https://huggingface.co/models?other=reinforce\n",
         "\n",
         "And you can find all the Deep Reinforcement Learning models here 👉 https://huggingface.co/models?pipeline_tag=reinforcement-learning\n"