If you can open up the back to get to the elements, a visual check will tell you quickly is one is open. That is the most common problem. You say it has
4 heat settings, so there must be a switch or a set of buttons to control that. Those should be checked if the elements prove to be OK.If the drier is okay then the other possibility is that the line is missing 1 phase. Measure each phase voltage to common/ground at the terminal block. Because the 2 elements are shared across each 120V phase, the medium setting might get 120V alright but the high setting also only gets 120V. This could be due to an open fuse/relay or wire, either at the main electrical box, or terminal block, plug, etc...
A*s*i*m*o*v