Been my experience that low beams do not come on with the high beams. You are see high beams in all four headlights.

Get a meter and troubleshoot the issue. The feed for the high/low low beams comes from the headlight switch to the dimmer switch. If it works in high then the headlight switch is probably not the issue. The dimmer switch then feeds either the low or the high beams. Check that with your meter, at the switch. If you have both there then check for both at the headlights. That will answer your questions.

Throwing parts at it hoping you get lucky is no way to do things.