I hope this is a simple question.
I have just acquired and successfully installed Dorico Pro 4. I still have Dorico 3 and Dorico 3.5 on my computer — can I delete them now that Dorico 4 is installed and working? Should I delete them, or is there any advantage to keeping them?
Yes, just as JBH explained, do it as you like, there is no problem either way. Only thing is, if you decide to keep old versions, you are discouraged from running them both at the same time, as that will leed to undefined behaviour.
One other potential reason to hold on to 3.5 is if you have any projects that require tempo adjustments with the tempo editor which is not yet implemented in 4.