[Sarteck]

Definitely DEFINITELY create a new site.

Back up all of your data, and then make a "test site" with the backup, upgrade that backup to vB4, and then use your Admin Control Panel (or other means) to disable ti site for everyone but you (and your Admins, if you have any others).

You will likely not "lose" any content--it will all still be there--but since your site is quite heavily modded, you will want to find replacements for all of your Mods before you go live.

You will also want to find replacements for any Styles you have, as vB3 styles will generally not work on vB4 without heavy modification (it's much easy just to find a similar-looking style made for vB4 and use it).

Once your "backup" site is up and running properly, and tested out as thouroughly as possible, plan for a day or more of downtime. Close your still-running main site, upgrade it, then apply all the modifications you made in the backup site to the new site.

From experience, I can tell you that a straight upgrade and then trying to work out problems as you go will definitely piss off your users, AND it'll piss YOU off, too.