You can equip all 4 perc slots for the barb
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.
Just to wanted to add that the perks all seem to work even if they are in a locked slot. So Ironwill adds +100 magic resist if its in the first perk slot and the same if it is in the 4th perk slot.
Fixed in #109. Open a new issue if still occurring.