You can equip all 4 perc slots for the barb

[Mth0dman23]

https://puu.sh/JFvVN/356c380dc6.gif

You can tell by the .gif what I mean. but let me describe it in words if I am possible:

When u equip a Perc on the first slot which is unlocked, you can drag it from that slot to the other locked percs. Redo it 4 times and you will have 4 percs.

Despite that: First time I opened the percs site and I dragged Axe Spec (which is was the standart perc it had selected) it set 4 percs at the same time on all slots.