Development for Halo 4 started not to long after Halo 3 was released, So I think time doesn't equal good content. There were a HUGE number of factors that 343 had to deal with in rather undesirable ways to meet their deadline of 2012.

The reason the new game is being pushed out is probably because MS wants a Halo game on the XBO soon, my guess is this game was in production while Halo 4 was, maybe not a lot but the script may have been written and CA may have multiplayer planned out a bit. I'm looking forward to it. :)