AE cant render as Mp4 -there is no option to

If you want a really good reason that you can't get a good MP4 (h.264) compressed file from the Render Cue I will give you two. First, it never worked well, not even close to well. Second, the Render Cut/Output module processes only one frame at a time so it is impossible for the compression engine to create the IPB frames that MP4 MPEG compressions rely on for quality compression and small file size. If you do not send properly compressed files with a good set of IPB frames to a streaming service (YouTube, Vimeo, Instagram, Facebook) they will recompress it with a sledgehammer and you will lose quality. A lot of quality.

What is IPB compression? An I-frame has luminance and chroma information in the frame. The color is compressed in blocks of 4 pixels, and luminance is compressed for every pixel using from the original pixels. A P-frame is a frame that predicts where the pixels are going to move by looking at the next frame and guessing where they move. Everything is predicted. A B-frame is a bidirectionally predicted frame where the prediction is based on the previous and next frame. These P and P frames are a lot smaller than I frames. The Adobe Media Encoder looks at the original Comp, generates a string of uncompressed frames, and then it compresses the I frames and predicts the P and B frames from the stream of original frames resulting in file sizes that are up to 1/3 smaller with a lot fewer color and compression artifacts. That's why MP4 (MPEG) H.264 compression was removed from the Render Cue. 

There is one advantage of using the Media Encoder that you are not thinking of. You can render a file while you continue to work in After Effects. There is no noticeable effect on the efficiency of AE or the responsiveness of the UI if your system meets the Minimum System Requirements. If you want to get the most work done in the least amount of time, invest in a background rendering system like Render Garden. Render time on a modest system will be cut in half. On my iMac Pro, a project that would take 15 minutes to render in the AME or 10 to 12 minutes to render to a production format using the render cue will render in about 3 minutes using Render Garden and give me both a lossless production master and an MP4 while still allowing me to work in AE. To tell you the truth, I don't remember the last time I stopped working on a comp to wait for a render. I have 3 BG rendering solutions, RG is my favorite. 

You have to understand that using Dynamic Link does not save time in rendering. In fact, it usually takes longer to render, and can take much longer to render than using AE's output module to render and replace with a suitable production format.

Why? Because AE will be running in the background doing the render when using Dynamic link on top of Premiere Pro running in the background rendering the rest of the PPro sequence. Premiere Pro's Dynamic Link is really only a time saver for extremely simple composites, graphics or similar tasks you can't do right inside PPro. If AE can't render a full frame preview faster than 3 or 4 frames per second render it. If you invest in Render Garden you won't lose any more time than it takes to cue up the render.

Thank you Rick. I have looked at the help files, and I do know how to add files to AME, but I think my question is more about trying to understand MP4. Is it a wrapper, or a codec? And when a client asks for it, what is the best advice? In the end I gave them an M4V file and they are happy, but I am still frustrated that I can't get my head around this.