There sure is an easy way to find out...

Load up a bunch of Seafires (T3s) and go run them up against a bunch of Tempests (T4s) and count how many of your Seafires return to deck and how many Tempests your Seafires shot down.

I know the answer already, but I'll let you experiment on this one.

I heard that T4 is way better than T3...

In fleet battle, T3s are better than T4s
becuz CVs can load more amount of planes when they load T3s so can provide longer scout support for BBs...
otherwise, in GB, T4s are better choice becuz GB normally ends earlier than HA or FB

this is what I heard.... :)

You heard wrong Oneshot. It's a foolish fleet who relies on fighters to scout and even more so if you use T3's (in leagues and HA's). They are slower and more vulnerable to T4's. The enemy fleet is going to hunt down all your planes and rape them because they are too slow to scout~drag.

This already happened in the previous Yo/Mo league and literally every recent HA.

hmm....
but in 'my fleet's battle experience, (not HA)
T3s were prefered by our fleet's CV players, because
If you micromanage T3s, you 'should be able to' avoid engaging with enemy T4s
enemy T4s can't chase T3s for a long time cuz they also have scout task to do.

actually I rarely saw dogfights in my previous fleet battles, CVs' first priority
is not win in dogfights, providing sight for BBs.
I think this basic tactics(focusing on scouting and avoiding contact with enemy Ftrs)
are the main reason of out fleets triumph(Good BB players are premise for this...)