To touch on the first part of your inquiry, I would say it depends on what you are doing with the PC and what loads on the RAM are when doing so. If you are often seeing more than 50% load on your memory with everything you work with (or play) open then IMO it is worth adding more.
In the case that you have 4X slots do keep in mind that having (4) sticks on your mainboard could cause issue with RAM speed (XMP) and may cause slightly less performance in games so far as FPS.
The DDR standards you are discussing 4 vs 5 are platform hardware specific. Your motherboard and 10th or 11th gen CPU cannot use DDR5, as mentioned above.