+4 is the maximum safe limit on armors. There generally should be no price difference between +3 and +4 except for the Elunium cost.
It depends on how much money you're willing to shell out. Personally, all my gears are +4, and I perform well enough in the WoE situation. Versus monsters, I really see no difference. But if you do have the money, there isn't anything bad by getting overupped armors. Plus if you decide to slot cards into them, overupped armors generally have a higher and better resale value.