So if were talking about story continuity and what game we would play first story wise, we should play Halo 2, ODST and then Halo 3. For the extra question though, ODST is not that hard and is about the same difficulty as Halo 3 IMO.

Should I play ODST before Halo 4?

You might need to play Halo 4 to understand a few things, but other than that, Reach and ODST are optional. ODST can be passed up because it doesnt really explain too much or have much of a place in the story, but its still a good game.

Is Halo 3 ODST better than Halo 3?

Sure, this years Halo game could be seen as something of a placeholder, a small side adventure to keep fans occupied until next years Reach. The graphics are nearly the same as those in Halo 3. So are the enemies, weapons and vehicles.