I have a 4600 same vintage. 4600 is the large frame, I think the 4200 is medium frame. So there are going to be some differences. I had the john deere 3 SCV put on by the dealer. It is a mechanically simple install- no rocket science. But a big PITA ergonomically- so someone more limber and willing to get oil in thier hair took my money. As long as you have the loader valve on the machine already the third valve is just sitting there in the loader valve block. all the kit needs to do is to get a control line and hyraulic fluid in and out of the block. The valve control is a bicycle brake type cable. this runs through a drilled hole in fender, to a new lever on the fender that replaces the cupholder. For my tractor the kit included the hard lines to the rear. It went into blank holes on the power beyond bracket. If you don't have power beyond, you will need the bracket. Then the dealer/factory "rube goldberg" part. This was for a front grapple, so soft lines from the disconnects were run to the FEL diconnect bracket ( Extra hole in an extension braket ).
Later tractors (4X10, 4X20 ??)have the optional lever near the 3PH lift lever for third SCV. And I think the diverter valve option on the 3X20,4X20 series lets you have maybe 2 or three extra functions ( Not sure). I have too much in this machine to " upgrade" machines before I win the lotto. Get the parts while they are still there. I had difficulty finding the complete kit for the fourth and fifith SCV's on my discontinued 4600 model. Repair parts would be lots of $$ to order all the pieces individually.