Board Game Arena

what happens if no heat left but wanna go from gear 2 to gear 4?

Last game I crossed the finishes line with 3 cards but wanted to play 4. I thought I might crash so only played 3 cards because I had no heat card left to discard. Now I wish I would have tried what would have happend. I couldn´t find anything about this case in the rules. Does anybody have any idea?

You just can't

I’ve just got this and the game won’t let me play

You can't. You need to have a heat to go from 2 to 4