Official comment
rekordbox 5 was a big step up in the features and functions. I understand that version 4 may have run better than 5 does on your computer, but with all due respect, your computer is 9 years old and only meets the minimum computer hardware requirements.
If you're trying to get the most out of that computer, my recommendation would be to create a second user profile and disable ANYTHING that you can get away with, and I mean EVERYTHING. If it's going to sap any resources, disable it, deactivate it, uninstall it. And when running rekordbox, turn down the graphics performance mode, don't run lighting, FX, or the sampler unless necessary, and certainly, never video.
Sorry, but you're trying to squeeze blood from a stone at this point - a nine year old computer doesn't owe you much, it's likely time to consider an upgrade.