Welcome to TBN! Are you buying a new 4120? With an aftermarket cab? Perry, Laurin, etc.?
The 485 backhoe will work with the factory JD cab. 'Course JD doesn't offer a cab with the 4120.
I'd guess that the 485 will work with aftermarket cabs - but not certain of that - no idea if the earlier model of JD backhoes - 448 and the 48 - will accomodate a cab; since the mounting frame for those models was never designed for a cab at all, whereas the 485 frame was specifically designed for use with a cab as well as the open station.
If you're buying a new tractor - you can expect to pay below MSRP as part of the tractor package for the backhoe, too.
Because the 485 is a recent addition to the JD CUT lineup; used backhoes are very few and far between... You might be waiting for a few YEARS to find a good deal on a good condition 485 hoe.
AKfish