Not to rock your boat, but T3 is a process, not a hardness:
That’s a very common caveat that folks fall into, you’re definitely not the first nor the last. Funny thing T351 is comparable to T4, and in some conditions T3 is superior to T4. I’ve been messing with material grades for years and I can’t say I’m any sort of expert, but I keep a few little webpage “helpers” bookmarked for this sort of thing. Another good one is:
You can register a free account which will allow you to compare up to 3 materials at a time, pretty nice!
Dan