I would strongly recommend you do not replace your vB4 forum with vB5. vBulletin 5 is not suitable for running in a production environment, you will find much worse performance and much fewer customisation options from third parties such as ours, and our vB5 mod is not in active development. It is also missing features compared to the vB4 version.
That being said, if you still want to go ahead, let me know and we can work something out.