DDR4 and DDR5 don't mix — and the slot won't even fit.

No.

DDR4 and DDR5 use different physical key notches, so the stick won’t go in. Even if you somehow forced it, the voltage and signaling are completely different — you’d likely damage the motherboard or the memory controller.

Hybrid laptops with soldered RAM usually lock you into one generation. If the soldered part is DDR4, the slot is also DDR4. If it’s DDR5, same deal. There’s no crossover.

Don’t force anything. Check your laptop’s spec sheet and buy the exact generation it expects.