I can't comment on GP4 compatibility (haven't played it for about 13 years) but I agree that the T300 is very good. What isn't good on the T300 is the pedals.
I bought the Ferrari Alcantara T300 Edition instead, which gets your the same T300 base, a very slightly larger wheel (Ferrari themed, but I can live with that) but most importantly the upgraded T3PA pedals (which can also be bought separately). OK it's more money, but for things like iRacing it's worth it.
Still I don't do much racing these days - the necessary practice was starting to wear thin.