sx.e upgrade
If you have custom code, it must be modified to interface with the new database schema (there are changes between 3 and 4) and the new code. If Infor/NxTrend wrote the mods for you, you will have to contact your SI manager there to have them do a custom code review. If you've modified them yourself, you will have to test them thoroughly prior to going live with it. You can also get a list of changed programs in the upgrade, this may point you to pieces of code that will have to be reviewed/updated. If you have sx.Architect, I recommend using that to do your own custom reviews.
Good luck, we all love doing upgrades!