I think it would be better to reduce the number of pre-defined single-report Gamepad instances to 2.

Having 4 does not make much sense IMHO as even on the most capable 32U4 there are only 3 usable endpoints, so besides barring the possibility to use the Gamepad with one or two different devices, the 4 instances cannot even show up all together.

Having two - or even a single one - is not a limitation, since additional devices can be declared in user sketches, if more are needed.