I've seen 33s on stock 1/2 tons. In tight situations they would rub for sure, but still fit otherwise. A 3/4 ton is already a little taller (by about an inch IIRC), so 33s would fit pretty well stock, maybe a little tight and probably rub in extreme situations but still fine in every day street driving- probably describes what you have going on. My 1/2 ton Suburban is lifted 2" and 33s fit quite perfectly. If you had a 4" lift, 35s would fit great, especially on a 3/4 ton. I once had a k5 that was 4" lift and took 35s just fine. Being a K20 you might be able to squeeze on a 36" or 37" tire, not sure if you would need to trim or not, but it would rub if you did any offroading or any other strenuous truck stuff with it.
TL;DR- 33s are a little too small for a K20 with 4" lift. Not horrendous, but would look a little small for sure. IMO 35" tires would be the perfect sweet spot for you. You may be able to push it bigger if you're a mall crawler and simply MUST have the hugest tires you can fit, but it would be silly as far as I'm concerned.
33's wouldn't be the end of the world, 35's would be basically perfect. I would personally say you shouldn't plan to go much bigger