When I bought my Olds a few years ago the low beams worked fine but one of the high beams didn't come on when I clicked the floor switch. I ignored it and assumed the bulb was burnt out. However, tonight I noticed all four come on with high beam, but only one light comes on out of four with low beam. Any thoughts? Bad dimmer switch (seems to function fine) or is there something going on somewhere else?

 

If any of the lights come on in both positions of the dimmer switch then it can't be the dimmer switch.
Check the bulbs , grounds , or wiring.
Only the two outside lights will come on in low beam.

 

Sounds correct to me
A H/L bulb's two filaments are independent and one can work while the other does not.

Filaments are like

LH H H HL

So, if you have one L filament out, on Low, you will show

_ _ _ L

or, on High:

H H H H

as the L filaments are not active when on High.

So, take a look at that dead bulb and verify BOTH filaments via testing with a battery or ohm meter. Or hotwire 12V and a ground to it and observe operation. You can find out what terminal does what by looking, examining the connector's wire colors and compare to other beam... or your friends on the internet.

Trust, but verify

 

Replace the headlight that won't come on.

 

You have a burned out lamp. It's that simple.


Replace that one and clean the contact prongs and sockets on all four.

 

Fixed with a new headlight (not easy to find) . Thing that threw me was all worked when on high beam - Octania's comments clicked. Thanks.