This rumor has sprung up from the resume of programmer Brandon Layton, who has been at work on Halo 4 since July 2011. According to his resume, his work on Halo 4 has listed the following duties (among others):
1. Wrote automation in C# that used in-game C++ hooks to create bots that fought each other to stress multiplayer.
2. Created automated bots using C# to systematically test multiplayer/campaign environments.
3. Custom made tool allowed anyone on team to run bots on their console overnight. Used SQL database to allow bots to run concurrently.
It's intriguing, but there is the very real possibility that the bots are being used strictly as development tools to help test maps, and are not intended for the product release, which is due late 2012.
The Halo series has always depended on enemy AI to provide fresh experiences and procedural action. It wouldn't been too much of a stretch for the series to finally include bots to flesh out unequal teams in mutliplayer, or to allow single player matches with bots.