As GARS is an add-on for vBulletin, I would need to see the vB4 code before I can say what needs to be done and how much time it would take, but from the looks of things, GARS might need a total redo of its templates, and possibly other things, as vB4 is going to have a "new front-end style marked-up with semantic XHTML and styled with CSS" (see
here for more info). Another feature that is going to be added to vB4 looks to be a "vBulletin Content Management System" (see
here for more info) but at this point there is no way for me to say if GARS will work with vB4 (probably not), what would need to be done to get GARS to be vB4 compliant (probably plenty), whether the vBulletin Content Management System makes GARS obsolete (could be possible), etcetera, without first seeing the vB4 code. The same holds true for the other geek scripts.