Don't get too hung up on this, but yes, you should be able to come up with an implementation that uses DMux. Step away from the problem, then come back and look at the specs for the DMux and the DMux4Way. How does the DMux behave based on sel? And how does the DMux4Way behave based on its sel bits?
As you progress you will be implementing new gates and systems that use previously built, simpler chips. You made a dmux, there is likely a way to do dmux4way using multiple dmux's. I found it very helpful to draw it out so you can visualize what you have to code out in the hdl file
It feels to me like you're thinking about the if/else nature of the specification like a programmer. Instead, let the internal DMuxes do the logic for you, instead of trying to do it with your Xor, etc.