I bought an HTC Evo off someone and they had it rooted, but for some reason 4G isn't working. I'm not sure if it's because they rooted the phone and it now has wifi tether (I don't know too much about rooting and ROMs) but could that be the reason why 4G isn't working? If there's a way to fix it I'd appreciate it.
Root access is unlikely the problem, but you can go back to stock and see what happens. Best and surest way back to stock is flashing a .sbf. The .sbf is the stock image provided by the retailer. If you're on Windows, you'll need RSD Lite to flash I believe. If on Linux or OS X use sbf_flash. You'll also need the .sbf for your particular device, but I'm not sure where the Evo images are; Google can probably help.
But again, I'd blame the network before an issue from root.
This walkthrough is the best I've seen on restoring an EVO 4G and removing root. It walks you through, step by step, and gives you everything you need.
And yes, rooting or ROMs could have caused the problem. If the rooting done in a way that manipulates the
Wimax partition and source, you may be in for trouble. Also, some ROMs simply do not support the EVO's 4G radios, such as the AOKP ICS ROMs.