I once attempted to socket a 3rd T4 materia and got the following message:

"You dont have the slightest chance of affixing that materia"

So maybe that would effectively be your cap? The second T4 had a chance of something like 16.5%, and as I said I was never even given the opportunity to try a 3rd.

If I remember correctly they were two HP+45s and a third HP+40