We have a guide on how to upgrade from Sycoin 3 to 4 here: 3.X.X to 4.1.X --- Upgrade Guide.
It’s not hard to use, I suggest you try it and post any questions that come up.
The guide doesn’t address which specific version you need to download, because that varies based on which version created your specific wallet.dat file. We should probably address what one can do if that version is unknown.
(Start with the latest (3.2.1) and try that on a copy of the backup, if it fails move to an earlier version and try again (always on a copy).
Using the guide and posting questions allows us to improve the guide and it keeps important information in visible places.
I increased the reply limit for posts on the first day temporarily so you can reply again.