Is the difference between m4v and mp4 just a matter of file extensions?

Is the difference between m4v and mp4 just a matter of file extensions?

No, there are other differences. For instance, the MP4 container can hold either MPEG-4 or H.264 video but the M4V containers is used exclusively for H.264 video. M4V files can contain 5.1 AC3 audio. M4V files can be DRM'd. Etc...

So if I had an .m4v that contained H.264 video and AAC audio (and with no DRM), could I simply rename the .m4v as .mp4?

You can always change the extension. In fact you could also change it to ".mov" and it would probably work. The problem is that some applications only look at the extension while others check the internal headers and if they find an inconsistency, would likely not play/synch/use/import the file as expected. For instance, I believe you would have problems using an MP4 extension on an M4V containing H.264/AAC/AC3 data tracks. If all you want is the H.264/AAC tracks wrapped in an MP4 container, then a better solution might be to use the "Save As..." option in MPEG Streamclip and select the MP4 container rather than the MOV file container. However, be aware that any incompatible tracks in the M4V file would not carry over to the MP4 container using this strategy. (I.e., audio and video tracks are simply copied to the new container avoiding re-compression delays.) It is up to you which method you wish to try.