Is fast DDR4 better than slow DDR5 for upgrades?

Usually. If you’re on a budget and upgrading an existing system, a kit of DDR4-3600 CL16 will outperform DDR5-4800 CL40 in most real-world tasks because the latency is dramatically lower. Cheap DDR5 has loose timings and low frequency, so you’re paying a premium for worse performance, especially in gaming. Only go DDR5 if you’re building a new system that requires it.