Compatibility comes first
A DDR5 board accepts only DDR5 modules, and a DDR4 board only DDR4. The notches differ, the electrical design differs, and no adapter changes this. Choosing the processor and board therefore fixes the memory generation automatically.
For any new current platform build, the answer is simply the generation the board requires. There is no meaningful decision left at that point, only capacity and a supported speed.
When DDR4 is still the right answer
If you already own a working DDR4 system, adding capacity or replacing a failed kit is often far better value than rebuilding the platform. A memory upgrade on a compatible board costs a fraction of a new processor, board and kit together, and in many real situations the capacity increase is what you actually needed.
Budget builds on older compatible platforms can also make sense when the price difference is large and the performance target is modest. What does not make sense is buying DDR4 memory with a DDR5 board in the same order.
What the generation change actually buys
DDR5 offers higher bandwidth and higher capacity ceilings, which helps most in memory sensitive games at high frame rates and in workloads that move large amounts of data. The gains are real but usually modest compared with the effect of the graphics card or having enough capacity in the first place.
Judge the platform as a whole: processor, board and memory kit together, against what the same total money would do elsewhere in the build. That comparison, not the generation label, is what should decide.
Working out the total platform cost
Compare complete platforms rather than individual items. Add the processor, the board and the memory kit for each option, then look at what is left for the graphics card. That single comparison answers most of these questions faster than any bandwidth chart.
Remember that capacity pricing moves independently of generation. A larger kit in one generation can cost less than a smaller kit in the other, and capacity is usually what you will notice in daily use.
If you are upgrading rather than building, the cheapest meaningful improvement is often more memory on the platform you already own, leaving a full platform change for when the processor itself is the limit.
