if i upgrade to gig 5 what happens to my instrument programs like melotron, piano etc
What do you mean by “instrument programs”? Do you mean plugins or are you talking about the specific sounds saved as part of a gig file?
You already have a Gig Performer 5 Pro for Windows license.
Since you can run Gig Performer 4 alongside Gig Performer 5, you can test in detail.
Note: make a backup of all your gig files (e.g. Gig Performer 4 gig files). Just in case.
I’m worried that i’ll have to re-install my plugins like mellotron, polymode, piano, I’m not worried about losing patches or sounds
No - that’s not an issue unless you’re also changing to a different computer.