The update from 4 → 5 will update all your leads, forms and campaigns, there should be no need to import them
If you’re experiencing errors when trying to process the update please search the forums for those specific error messages as it’s highly likely someone before you had that issue
If you’re encountering errors without any previous reports on the forum, please make a specific post with the error and what you’ve done to try to troubleshoot in Product Support and then folks can help you get to the bottom of what’s going on.