export animation in mp4

Sometimes MP4, or Mpeg4, can be referred to as H264, so don't be confused by the different labels. I won't go into a lengthy explanantion of the differences here, but typically if you're rendering for online presentation you'll choose H264.

After Effects doesn't render MP4 natively any more, so you have to export using Media Encoder to do so.

There are two ways to send content to Media Encoder:

File > Export > Add to Media Encoder Queue

or

Composition > Add to Media Encoder Queue

Then, in Media Encoder, choose H264 as the output format, and select a preset based on your requirements.  I find that "Match Source - High Bitrate" is the most common preset I use, but your requirements may vary.