[JRTJH]

Turn the light switches on, use a volt meter to check for voltage at the bulb sockets. If not present, remove the light fixture and check for voltage at the fixture/wiring harness connection (probably a crimp connector) if not present, trace the wire back to the closest operable light in the circuit.

Since you say there are 4 lights on the switch and when turned on the first and fourth lights (closest to the switch and furthest from it) work, then it's probably a fixture/socket problem or a crimp connector for those two fixtures.