Why do you "need" to if you're only replacing a 2630 in a sprayer? 

Section Control in a 2630 was $3000 or $3500, so paying $850 a year will still take a little while before it's more expensive. But you also get Row Sense at that point, which on the 2630 would be another $3k or so....and data sync which isn't on the older displays. So in reality it will take quite a few years before the 4640 is more expensive than a similar 2630. Good luck finding a new 4640 right now, so hopefully you don't need it soon.