Cap is +4 sets in PvE. Therefore, if you already have legend affix +2 sets on ring and amulet, then there will be no difference.
Let's pretend you have no +2 sets on your ring or amulet in PvE. Now let's pretend you have one gear with Reactor set giving Reactor (1). If you had Reactor sets on Helmet and Mainhand in the same scenario then your gears would show Reactor (2). Now, let's pretend you add Elixir mythstone to your Ring for +2 Sets. Now, your Reactor would show Reactor (4) --> Two from Elixir and Two from Reactor sets. Following that thought trail, now you add a Nadroji amulet with the coveted +2 sets legend affix. Now, your Reactor would show Reactor (6) --> Two from Nadroji +2 Sets, Two from Elixir and Two From Reactor Sets. Now, let's pretend you have all the gears from the previous exercise and you now add Elixir to your Nadroji Amulet. This would make your Reactor (6), you cannot go over +4 PvE cap. You would need 4 Reactor gears plus +2 sets on Ring and amulet to obtain Reactor (8) --> 4 Reactor gears, +2 Sets Ring and +2 Sets Amulet.
I hope that helped out.