Restrincting permanent items to 4, NEed help!

My hero arena is working fine and everything but I am in need of a trigger that restricts the access to permanent items. to encourage the use of charged, well the problems are:

1) I can't use the TFT classification since there are Artifacts that are permanent and purchasable permanent items too.

2) I can't use hit points either cause I have a trigger that keeps taking out hit points from the items dropped in the arena.

3) Can't use item level cause from it depends when an item is dropped.

Sounds like you might have to use brute force.

By that I mean a massive trigger that checks exactly what item a Hero just acquired and knows if it is permanent or not. Then just keep a tally of how many permanent items the Hero currently has and if this item is number 5, force them to drop it.

Depending on the number of items available, this suggestion may or may not be feasable. Hope it helps, tho. :)