4 piece set bonus

My guilds going to be raiding heroics soon and they want me to get my 4 piece set bonus, for reasons I understand, so that it will be easier for us to progress. Problem is, Im so used to getting just the BiS pieces that I have no idea what I should substitute or what methods to find out this info. Any help is appreciated.

Typically for 4 piece set bonus its shoulders+chest+hands+legs. This minimizes your non survival stats gained (hit/exp) to gain the set bonus. If you want a more in depth answer on why those pieces I'm sure someone else could provide it.

Just look at the gear guides. Some tier pieces are BiS; those obviously make up part of the best four-piece set. For the others, look at how far down the tier piece is on the BiS list, and look at the comments about the pieces.

For the current tier, yes, you'll find that the helm is in general the worst piece, and the one that gets skipped. But if you take a critical look at the lists yourself, and learn why the helm is worst (compared to other options) then you'll be set to make this decision again and again for each tier.