Hi
I've been happy with my 990. It has excellent traction and power. The engine starts easy and runs smooth. The 990 is a great product. The tall tires and weight add up to a lot more tractor than a 790. The price, availability, and time was right for me. That doesn't mean that I'd never consider a 4XXX unit for the right deal.
1) How do you feel about working with a geared tractor without the bells and whistles ?? Did you get an opportunity to drive one of each ?? Even if the price is right, you won't be happy if you experience regret later, wishing you had the more modern unit.
2) What is the potential resale of a 4510 with a gear trans? My impression is that most folks shopping for a 4000 series will be looking for a hydro unit. If I was comparing, I'd shoot for a 4610 or 4710 hydro with the larger loader, but the price does creep up....
3) Buy what you like at the price you like.
Also, there seems to be more accessories, like soft cabs and such, for the 4XXX series, if you're interested in that.
Just my 2 cents from shopping last year.....
Enjoy,
John