To give you an idea on what the 2 bonuses do for your DPS, below is a sim showing the exact same gear sets, but the 2 & 4 set bonuses have been disabled.
As you can see, the 4 set adds around 8% extra DPS when dressed in BR Mythic BiS. Below are some simulations showing someone dressed in HM Mythic BiS and if you were to replace your head & chest piece for the 2 set, or the head, chest, shoulder & legs for the 4 set on each of the different gear levels to make a set bonus.
As you can see, it is worth it to get the set bonuses via normal gear, even if you are fully mythic geared and especially if you are still pushing mythic or HC.
A few notes about this sim:
The sims have been run using SN, RoP, PC as the talents with 450 sec (+/- 20%) fight length. Single target & light movement.