If I play this on a Truck, and later in the turn it flips so it’s no longer a Truck, what happens to its attack?
[Source: WotC]
If the character stops being a Truck, 4-Wheel Drive stops applying, and its base attack will revert to whatever is printed on the card.
Changing a character’s base attack is just like changing the number that’s printed on the card. From there, all other bonuses apply as normal. For example, if you have a Truck with 3 attack upgraded with
Indestructible Sword (that gives it +2 attack), its attack will be 9 (7 from 4-Wheel Drive +2 from the upgrade). If it stops being a Truck that turn, its attack will be 5 (base 3 attack +2 from the upgrade).