Noobie (me) wants to upgrade to 4.0 but doesnt want to lose anything
 

OK I want to upgrade to 4.0 but I dont want to lose my plugin settings, plugins, custom style (which is a version 3.7 btw), etc. (pretty much everything)
What stuff will be lost if I upgrade and how can I pack it up/save it so that I can add it back in?

Your style will be gone in its entirety, never to be used again. You might be able to pick pieces out of it, but assume that it's gone.

You are advised to disable all plugins, as the code is different, and existing ones can cause errors under vB4.0.

Non of 3.7 hack isn't gonna work on vb4 uless it's not updated. For more info check vBulletin 4 mods/hacks section and see if those hacks are listed there. If yes you should be good. No then don't even think about it.

Good Luck ..

This is not true. Some plugins for 3.5.x work with 4.0.

This is why I say 'existing ones can cause errors'.

Most of 3.5. X doesn't work with even 3.8.X ... I wasn't aware of it .. But strange.

In general none of 3.x will work for 4.x styles or modifications.

then ofcourse i know there are a few that actually work from 3.5,3.7,3.8 but so few
its hardly worth mentioning, and can hardly really be considered a modification.

tip 1 : if you want to have your vb4 site maintain the same style as now
create a local server with vb installed, and create a style looking excaktly like your current.
graphics can possible just be copyed over, so you just need to gothrough the css and stylevars
depending on the style ofcourse.

tip 2 : modifications, same as a above, except it requires alot of work to get
existing vb3 mods to work on a vb4 board.

tip 3 : local server with vb4 running and take it one step at a time, when happy with it
upgrade the live, and import the styles etc from local.