You won't lose any towing ability with 4 inches less in the back. There are a few ways to get the 2/4 done. Pull the 2 inch lift blocks sandwiched between the leaf springs and the axle and replace those U-bolts with the 2wd length version, swap the rear leaf spring shackle for versions that offer 2 inches of ride height drop... congrats, you're 4 inches lower in the back and all you need to do to ensure ample amount of wheel travel is cut or remove the axle bump stops and get some shock extenders if you don't want to spring for reduced ride height shocks. For the front, simply track down some Silverado HD 2500 torsion keys, also known as purple keys, and swap the torsion keys. I bought mine off of ebay for 25 bucks to my door when everyone else wants 50 or more for them. These keys drop the front 2 inches and then you can still crank or decrank the torsion bar key bolts to dial in the ride height up front to how you want it. I am a few weeks out from doing that torsion keys swap myself, but I have my rear down 4 inches at the moment.
You can find a picture of mine a few threads below this in the "pics of dropped OBS 4x4" or something like that. Mine is an NBS but I was asked to supply a picture in that thread and did, so go hunting in that thread and you will see some pictures that will help you out.