Here we have  a pic of  a handout from E3 that shows the default Halo 4 button layout.

Looks pretty straight forward to me. Finally the use of the D-Pad.

I do hope (and assume) that there will be other button layouts available. Hopefully, we can program our own layout. I prefer the H3 Default/Halo Reach Recon layouts myself.

Still, for long time Halo players, this should be easy to figure out and use.