Currently, if you introduce a new season of gear into your kit to replace prior season, you lose the set bonuses when mixing the old with the new. Beginning with Arena Season 3 on retail WoW in TBC, the set bonuses would work regardless of what season gear you used. e.g., you could have two pieces of S2 & two pieces of S1 (or any combination) and both the 2 and 4 piece set bonus would work. This would only work for pieces of the same set, i.e. all are Dragonhide (for druid). If you had 3 Dragonhide and 1 Kodohide, the 4 piece bonus would NOT work.
Currently, if you introduce a new season of gear into your kit to replace prior season, you lose the set bonuses when mixing the old with the new. Beginning with Arena Season 3 on retail WoW in TBC, the set bonuses would work regardless of what season gear you used. e.g., you could have two pieces of S2 & two pieces of S1 (or any combination) and both the 2 and 4 piece set bonus would work. This would only work for pieces of the same set, i.e. all are Dragonhide (for druid). If you had 3 Dragonhide and 1 Kodohide, the 4 piece bonus would NOT work.