Well, there is a lot waiting for you. Moving Majors happens from major to major. So from 4 to 5 and from 5 to 6 etc. Mendix 4 is structurally very different from Mendix 9. I think it is worth considering rebuilding parts in Mendix 9 because migrating can really take a lot of time and effort.
Information on how to move from version to version is available on the forum and on docs.mendix.com. For example this: https://docs.mendix.com/refguide/moving-from-8-to-9
Keep in mind that this is not going to happen in 1 day and maybe not even in 1 sprint. Every time you do a Major upgrade you should do regression test and make sure everything works. For example, if you've built Custom JAVA actions, you'll probably need to make them work again, piece by piece. Modules that you have downloaded from the Marketplace must also always be upgraded from the app store. one by one.
This way you go up a version step by step! Goodluck.