When will a Halo 4 download be available

when is there going to be a download for the halo 4 game on the xbox live marketplace? my xbox’s dvd drive is no good and I can’t put a new one in there because i don’t know what model dvd drive was in there originally so i have to download all the games i want to play. I have always been a halo fan and would like to play sometime in the near future. when will that be an option?