I want to be able to export mp4's directly out of media composer on a Mac.

H264 Mp4 is the most compatible video file format for client approvals and it would be great if I don't need to use another encoder software for that. 

Just export to mov and change the extension to mp4

I think changing the extension of the file is not a real rewrap with writing the header information. So it can make problems on some devices / software and it's not what you want for a client approval. 

You can export to .mov with the specs you need and then use Quicktime 7 to to convert to .mp4. Just make sure you change your video and audio settings to Pass Through so there won't be an added layer of compression. It bascilly just rewraps your Quiciktime to mp4.

By the time Avid adds the ability to export video as H264 wrapped in MP4, H265 will probably be standard.

Handbrake and QuickTime 7 are free and easy to use, that's what I use as a workaround.