I am about to purchase a work machine, which I'll use for Office apps, teleconferences, web browsing, and a bit of 2D autoCAD. How much difference is there in the performance of a 3rd gen and 4th gen processor that I would notice as a non-gaming, non-video watching user?

http://www.tomshardware.co.uk/...gen-intel-processors.html

most likely you wouldn't notice difference if both have same speed.