Why do some vB 3 mods work for vB4

[New Joe]

I was wondering why some Mods work on vB4 which were made for vB3?
Here is one example: Multiple account login detector (AE Detector)
https://vborg.vbsupport.ru/showthread.php?p=2008598#post2008598

Now I have tested the Casino Mod on vB4 it kind of works but with some errors.
Really wish it could work for vB4, as this is one of the few reasons I'm not going live with vB4 on my site, as this is an important Mod for many of my users.

[Lynne]

If a mod is all php and is using hooks that are still valid, then chances are it will work (unless it has queries that have to do with attachments or the search tables which have both changed). It's just whenever you have a mod that uses templates or template changes that you will have to upgrade it.