The 4690 backhoe was a mainstay in the Kubota product line for over 20 years, from 1981 until into the 2000's. It was a 3 point backhoe, meaning you attached it at the implement points rather than installing a separate subframe just for the backhoe. As such, it was meant for lighter duty service since the linkages would expose the unreinforced tractor to considerable stress and torque if used for heavy digging. The 4690 was spec'd for the B7800 at one point, along with the later subframe BH75 backhoe. Here's a scan of the brochure I had from years ago with the 4690 specs (with copyright acknowledgement to Kubota... I hope they don't mind).
I wish I could simplify the model numbers. Tractor companies like Kubota and Deere have so many model designations, it's impossible to keep them straight without looking each one up. Don't hold me to this but the earliest BH's had 3 digit model numbers, followed by the 4 digit model numbers that were used from the 1980's until about 2003, that were, in turn, replaced by the current series of 2 digit model numbers. Within each series, though, you can't tell anything about the size or 3 point/subframe configuration by the model number.
The backhoes also have a confusing series of prefixes and suffixes that I've never been able to figure out. The "K" prefix is apparently for backhoes used on the larger "L" series tractors, not for a "B" model like you have. If you find the "Prior Products" section of the Kub website, it lists what years each model tractor was produced and on the "Implements" tab it shows which loaders, backhoes and mower decks went with each tractor.
Two vital points about that 4690 backhoe. Because of the 3 point configuration, it's necessary to install a reinforcing bracket on the top link mount of the tractor. You either need the one made to fit your B7800 or you need to fab one up. Digging with a backhoe puts extreme force to the attachment points on the tractor (hence the reason why a subframe backhoe is preferred). Unless you reinforce the top link bracket, you run the risk of breaking the rear end of the tractor under heavy digging.
The other rap against a 3 point BH is risk to the operator if something in the linkage lets loose. The operator is sitting right under the ROPS. If it fails while digging, the whole apparatus can come up off the tractor in a catastrophic manner. It's safe if installed and used with proper care, but don't take chances.
