m4a mp4 file format what's the difference or are they the same?

Question

I've looked and can't seem to find a concrete answer.

My client wants to play sound clips on his website, currently he has over 2000 clips which are in m4a format.

A lot of the audio players I have looked at say they support mp4 but no mention of m4a.

From the wiki on mp4 it says they are both the same except that m4a is a container for mp4 or something along those lines.

So my question is can i play m4a files with an mp4 player?

The alternative is for my client to convert all his files to mp3.

What's the best way forward?