I'd buy the 4GB model in whatever color you'd prefer. The 8GB model isn't worth the premium when you consider you can stick a 128GB microSD card into any of the Sansa Clip models–a measly 4GB is meaningless at that point.
Also, converting .m4as (that's what Apple uses, right?) to .mp3s isn't necessary with Rockbox–in fact, I believe the native Sansa firmware handles .m4as no problem.
I strongly recommend installing Rockbox on your Sansa Clip. You'll have better, more customizable sound (you can change all the EQ settings to suit your preference), better battery life, easier navigation, and almost unlimited options to customize how your device works (user interface, brightness, time out, locking functions, playback functions, etc).
The native Sansa firmware is really slow and clunky, especially when it comes to navigation and updating your file database (which happens every time you connect/disconnect it from your computer). I have a [not full] 32GB card in my Sansa ClipZip right now and it takes the native firmware about 10-15 minutes for it to rebuild my database when I disconnect it. I believe the Sansa firmware has an upper limit of about 8,000 songs too, so unless you Rockbox your Sansa, you'll be limited to about a 32GB memory card.
There are other mp3 players (none of which are particularly light weight) on the market that take multiple microSD cards if you're pushing the 128GB limit.