I'm installing the hydraulic thru-hull studs onto my new k-plane transom plates and I'm wondering if I need to put any sealant on the threads that pass through the transom plate? Basically I'm screwing the stud (that serves as the hydraulic pass thru) from the back side of the transom plate. Do I need any sealant on these threads? If so, what kind?

Yeah. 4200 works.

I wouldnt put 4200 on the threads, but where it passes thru the transom yes.

What should put on the threads on the hydraulic studs that screw into the k-plane transom plate? If water passes through this area, it does not matter how well the 4200 seals between the k-plane transom plate and the transom.

There is no harm in putting 4200 on the threads. It is a 3/4 inch rough thread, the 4200 seals and helps to keep the steel fitting from galling the aluminum plate. You can use any sealer you want, personally I prefer 4200.