Can I reuse DDR4 RAM coolers on DDR5 sticks?

Probably not, and definitely not worth the hassle. DDR5 sticks have a different PCB layout with the PMIC and voltage regulators on the stick, so clip-on DDR4 coolers often won’t clear those parts or could cause shorts. Even if they fit, DDR5’s different heat distribution means the cooler won’t help, and most DDR5 sticks already come with decent heatsinks. Save your money.