You can actually move gear over to the offset slots and thus equip 5 items, though they no longer work if not in the proper slot anyway (not an exploitable bug).
I am like 90% sure this is because TFC injects an extra slot for the back item and the gui is probably not accounting for that. Going to have to see if it can be fixed with a mixin.
See image below of offset gear slots.
You can actually move gear over to the offset slots and thus equip 5 items, though they no longer work if not in the proper slot anyway (not an exploitable bug).