T18 4P: This bonus works a little strangely, because it rounds the stacks up and does not keep track of partial stacks. What this means is if you have 1 Scent of Blood stack, and you Death Strike, you go to 0 stacks. If you have 2-5 stacks, and Death Strike, you go to 1 stack. If you have 6-10, you go to 2 stacks after Death Striking. What this means is to maximize this bonus you have to 'game' it. Get to 2 stacks of Scent of Blood. Then Death Strike down to 1. Then Blood Boil or Soul Reaper to get back to 2. Death Strike again down to 1. Ect. This way you are affectively making this bonus retain 50% of your stacks instead of 20%. The problem is this means you have to perfectly alternate DS and BB/SR. Keep in mind Rune Tap doesn't generate stacks for example, so using that can mess up your 'rhythm', or having to DS twice in a row for some reason. If you don't game this bonus like this, then it basically does almost nothing. However if you do game it perfectly, it effectively generates an SoB stack every Death Strike, which is actually pretty decent. This bonus isn't game breaking, and honestly wearing 4p is going to cost you stats, so it's kind of subjective if it's even worth wearing, but if you do have this bonus, make sure you use it correctly.

Summary: You should get the 2p if you can, the 4p is more questionable, if you don't 'game' it, it is almost useless. Offpieces + 2p can actually give more mitigation then wearing 4p in some cases.