It depends a lot on the room. In a bat cave fully light controlled room with dark non reflective surfaces contrast trumps resolution. The HD33 is not an overly bright projector so you must have fairly decent light control. I had an HD33 and loved it especially because it had excellent FI but many are not a fan so if you don't use it ,it's not a plus. Unless you go the refurbished route $1500 is not going to get you a higher contrast projector, try and snag a refurbed Epson 5040. If your room is less then ideal like a white ceiling contrast is all ready compromised so a BenQ HT3550 might be an excellent upgrade with an msrp of $1500 but may have too short of a throw so use the calculator to check it out. Upgrading to 4K needs every thing in the chain so HTPC, AVR and cables need to be up too snuff. If streaming 4K a 25 Mb ISP is the minimum. Upgrading a PC I recommend a NVidia 1050ti card at a minimum.
"Smart enough to know better, to old to care" ------ Dedicated Bat Cave Home Theater, JVC RS49U/Mitsubishi HC7900DW Projector, 110" 16:9 Jamestown screen with variable power masking for CIW 2.50:1 to 16:9, Marantz 7009 with 7.1.4 Atmos with Ohm mains,3 DIY Subs (2 15" (1 ported, 1 sealed and a 12" 4th order bandpass), 1 DIY butt kicker, Custom Built HTPC, 18TB DroboFS NAS