I have an Opemus 6 B&W, which doesn't have a transformer. First thing I would try is to plug the enlarger directly into a known working mains socket, without a timer in between. Next, test the mains plug fuse. If you have done that, check the connection box on the outside of the lamp housing (you may not have this, I'm not familiar with the 4), mine has screw terminals that connect the mains cord and internal wiring. Make sure the wires are all firmly attached and no stray strands. After that you will probably need to test the mains cable through to the lamp socket for continuity.
Ian