Edit: Actually I'm going to assume you're a warrior because that's what I am. Generally people prefer 4pc T10 and Pillars of Might for the legs. The 4pc bonus is really nice for the additional survival cooldown.
I am assuming you are a Warrior cause that would just be a safe bet considering ... But even if you are a Paladin or DK the same would probably apply (not sure on the DK part). There are two pieces of offset gear that are head and shoulders about the tier equivalents in my opinion (no pun intended). So it only matters of which peoples would you be swapping in?
The 4set bonus is nice but completely unnecessary in a world of 30% buff. It is purely situational. We do not know the rest of your stats so to me the major consideration is if you have a decent level of expertise in other slots and if you have enough +hit elsewhere as well. You honestly can not ever say "Only go to 4set at 277" without considering your other equipment. Dropping down to 0 expertise is never a good thing.
Dropping down to 0 expertise isn't really a bad thing... and it's pretty impossible since you get some from talents. Expertise is helpful on Sindragosa Heroic and... that's it! Oh yeah, if you don't have the threat output to stay ahead of dps. That being said, tanks have cleared all of the hard encounters with very little expertise and still not had trouble keeping ahead of the dps. Rotation is key, and remember that Shockwave can't be dodged or parried, so it is awesome if your expertise is low. It's great anyway.
Warrior/DK: Go ahead and swap over if you can manage without the expertise. Or, just wear the 2-piece for the farm bosses (more threat, more DPS, boss goes down faster) and then swap over to the four piece for the hard stuff.
Pally: Never, ever, ever aim for the four-piece. It is complete trash. I would slap Blizzard for giving us dodge, if I didn't remember how ridiculously OP we were a patch ago.