At work I have a 2 x quad extreme 3.33 w/ 8 gigs, doing windows dev for a large medical devices corp where at any given time may have to run multiple copies of Visual Studio, a db server, application server, multiple virtual machines simulating inventory cabinets, source control, and bug tracker.
At home my main computer is now a base 11" MBA, primarily used for hobbyist dev/learning. It seems absolutely fine, but I don't do Java/Eclispe, right now primarily RoR, scheme, and iOS. I'm usually in VIM or XCode with some web pages open utilizing 2 or 3 spaces at a time, and regularly have 500mb to a gig free. Soon I'll be playing around with JVM languages such as Scala and Clojure.
Normally I'd agree on getting 4gb without a second thought, but as someone mentioned above Apple's upgrade scheme here is a bit out of whack. I decided to see if it's really necessary and it surprisingly hasn't been for my usage. I guess I wonder if you guys saying 4gigs is a must have instrumented this constraint, and what exactly were you running? RE: comments about "you can't upgrade it..." well sure, but if it truly is a problem resale on Macs is fantastic. You're not marrying the machine. If it's _sorta_ a problem then you're in an interesting situation where maybe you can learn to manage constraints and streamline your workflow, which are hallmarks of being a great engineer. It might be a worthwhile experiment, esp. if you defray the cost by going refurb.