Would not let the ROPS thing stop me if it was a decent deal on the 4310. These are solid, reliable tractors that will last for many years.
If you want the 9 ft. factory BH, yes, they would change the ROPS. The factory BH may still be available new from Kubota; a new ROPS should be part of the package offered by the dealer.
Some excellent 9 ft. subframe backhoes also available from other mfrs, like Woods, Bush Hog, Bradco, to name a few, that should also fit a 4310 like a glove.