I installed a splendide combo a few years back, and I was nervous about drilling a 4 inch hole as well. But what I ended up doing was measuring on the dryer where the hole would need to be, drew a 4” hole on the inside of the RV wall, and drilled a small hole in the very center of that 4 inch hole, making sure not to go so far as to penetrate the outer skin. I then popped a bunch of similarly-shallow holes from that center hole to the outer marking, making sure that I didn’t hit any wall studs, plumbing, or anything else. So I had a 4 inch hole drawn on the wall full of quarter inch drill holes. Once I was sure it was safe to drill there, I then drilled the one center hole all the way through to the outside. Then I used a 4 inch hole saw, and drilled in word from the outside.
One way to do it, anyway. Good luck!
2010 Fleetwood Southwind 36D
2003 Jayco Greyhawk 27DS