Hello there! Thanks for posting your TensorFlow.js question and welcome to the community  Indeed you may be interested in this codelab that shows you how to make your custom image classifier via a form of transfer learning using mobilenet as the base TensorFlow.js Transfer Learning Image Classifier  |  Google Codelabs 18
This is how the popular Teachable Machine (Teachable Machine 7) works behind the scenes to take video and retrain a network on top of mobile net to then recognize something new and all the retraining is done in < 30 seconds in the browser live if you have say 50 images of the new class or so.
If you wish to use MP4 files then you can simply take the frames of these if they only contain the new object you want to recognize and then use those frames as the training data.