DDR4 and DDR5 don't mix.

No. Not in the same system, not at the same time. They are physically and electrically incompatible.

The slots look similar, but the notches are in different places. You can’t even force a DDR5 stick into a DDR4 slot (and if you managed to, you’d fry something). Beyond that, the voltage regulators and signaling are completely different. The memory controller on the CPU is designed for one or the other, not both.

Some motherboards exist with both DDR4 and DDR5 slots, but you can only populate one set at a time. The manual will say “DDR4 or DDR5” – not “and.” Pick a lane.

Future-proofing or budget building? Pick one and stick with it. Mixing isn’t an option.