Is the keyboard version upgradable to an Ultra? I thought it wasn't but maybe I'm mistaken. I'll second exactly what Digiboy has said.
There is another difference too. The Ultra's have slightly better midi timing as well so you if you use your E4K for lots of sequencing, you may find that the Ultra will make your sequences sound tighter. That's not to say the non-ultras are bad though.
If it is doing the job, why spend the extra money. IMHO though, spending an extra hundred or two over the $1195 will get you an E5000 Ultra to add to your E4K. That would give you double the available amount of RAM, extra polyphony, extra outs, Beat Munging, etc. and a full warranty. I think that that is a better deal than Ultracizing your E4K.