Are you talking about handling or roadholding/grip?

Given 2 cars with the same size tyres on and all intents and purposes the same?
If the 4wd car tries to put the power on earlier out of a corner, i.e. while the tyre is still subject to lateral input or "cornering force" it's probably going to "work" it's tyres less as the longitudinal inputs or motive force is applied - the same amount of power is being dished out to four wheels so the overall forces on the driven wheels will be less. In effect, you are less likely to exceed the overall tractive forces of the tyre i.e. break traction in a lateral or forward direction.

Tyres on a car don't become blessed with more grip if they are on a 4wd car.

Handling is a bit of a different thing