Change links unit 4

README.md

@@ -27,7 +27,7 @@ This course is **self-paced** you can start when you want 🥳.
 | [Published 🥳](https://github.com/huggingface/deep-rl-class/blob/main/unit2/README.md)| [Q-Learning](https://github.com/huggingface/deep-rl-class/blob/main/unit2/README.md)                                               | [Train an agent to cross a Frozen lake ⛄ and train an autonomous taxi 🚖](https://github.com/huggingface/deep-rl-class/blob/main/unit2/unit2.ipynb).                                                                                                        |
 | [Published 🥳](https://github.com/huggingface/deep-rl-class/tree/main/unit3#unit-3-deep-q-learning-with-atari-games-) | [Deep Q-Learning](https://github.com/huggingface/deep-rl-class/tree/main/unit3#unit-3-deep-q-learning-with-atari-games-)                      | Train a Deep Q-Learning agent to play Space Invaders using [RL-Baselines3-Zoo](https://github.com/DLR-RM/rl-baselines3-zoo)                                                    |
 | [Published 🥳](https://github.com/huggingface/deep-rl-class/blob/main/unit3/bonus.md)| [Bonus: Automatic Hyperparameter Tuning using Optuna](https://github.com/huggingface/deep-rl-class/blob/main/unit3/bonus.md)| |                         |                                                    |
-| [Published 🥳](https://medium.com/@thomassimonini/an-introduction-to-unity-ml-agents-with-hugging-face-efbac62c8c80) | [🎁 Learn to train your first Unity MLAgent](https://medium.com/@thomassimonini/an-introduction-to-unity-ml-agents-with-hugging-face-efbac62c8c80) |  |
+| [Published 🥳](https://github.com/huggingface/deep-rl-class/tree/main/unit4#unit-4-an-introduction-to-unity-mlagents-with-hugging-face-) | [🎁 Learn to train your first Unity MLAgent](https://github.com/huggingface/deep-rl-class/tree/main/unit4#unit-4-an-introduction-to-unity-mlagents-with-hugging-face-) | [Train a curious agent to destroy Pyramids 💥](https://colab.research.google.com/github/huggingface/deep-rl-class/blob/main/unit4/unit4.ipynb)  |
 | [Published 🥳](https://github.com/huggingface/deep-rl-class/tree/main/unit5#unit-5-policy-gradient-with-pytorch) | [Policy Gradient with PyTorch](https://huggingface.co/blog/deep-rl-pg)                                     | [Code a Reinforce agent from scratch using PyTorch and train it to play Pong 🎾, CartPole and Pixelcopter 🚁](https://colab.research.google.com/github/huggingface/deep-rl-class/blob/main/unit5/unit5.ipynb)                                       |
 | [Published 🥳](https://github.com/huggingface/deep-rl-class/tree/main/unit6#towards-better-explorations-methods-with-curiosity) | [Towards better explorations methods with Curiosity](https://github.com/huggingface/deep-rl-class/tree/main/unit6#towards-better-explorations-methods-with-curiosity)|                                                                                                         |
 |  July the 15th | Actor-Critic Methods                                     | 🏗️                                                                                                        |