Isn't it simpler to just make 4 copies of the ability, each at a specific level, and with an exprreq to require the last one?
If not, what's stopping you from just making the ability non-unique, with a maxlimit of 4, and leave it up to the user to set the right level? (write an eval rule to complain if there's more than one copy of the same thing at the same level)
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