Nah, I aint that smart - I just hang around here too much and glean a lot of info...
FWIW means For What Its Worth. I was really nitpicking a bit when I pointed out it isn't a subframe. Seon is right when he describes the RSA system. Those side plates are nearly 1" thick. It is a very stout mount.
The real thing is it isn't a 3 point mount. Most dealers shy away from those, because there is (in legend at least) a lot of tractors that have been wish-boned by them. There ARE happy 3 point backhoe owners on tbn. Personally, i wouln't have one... I am the kind of guy that once i hear of a potential problem, it always happedns to me!
Anyway. a subframe goes UNDER the tractor. Some machines need that to support the backhoe operation. These sometimes interfer with the mid mower (some machines even have it that you can't use a mid mower at all if you use their subframe backhoe - not even with the backhoe off...) Now, no one is going to run a finish mower with a backhoe mounted, but with RSA you could...
Why is this nitpicky? If by subframe mount, you are talking about a structure mounted under the tractor's 'frame' than RSA ISN'T subframe. But if by subframe you mean 'a good strong mount that won't tear up the tractor' then RSA IS.
One last caveat. Can a 4210 take a 47? My supect memory tells me that Deere didn't rate them together. Why, I don't have the faintest idea. It may just have been marketing. OK I remember something. Early 4210s had smaller hydraulic pumps. Later machines had beefed up ones. It might be that the hydraulics were less than ideal. But for the right price, you may be very happy with the results.