I've got to think about that.........but has someone recently installed a single pole switch where a three way should be.

Otherwise, I'd have to guess at your exact configuration ( ie: exactly what you have in eech of your switch boxes.)

Maybe someone will know exactly what the issue is....but I think they will need the same info.

 

I'd start by figuring out where the 3-ways are and where the 4-way is. It matters.

Here's a basic schematic of how 4-ways work.

 

Location of dimmer and is the dimmer a 3 way dimmer.

 

Is the dimmer a 3 way dimmer. Did this ever work?
Tell us about the wires at each switch. Omit grounds. Note the common connection not the location on the switch.

 

It worked once & now doesn’t: switch failure or wiring failure.
It never worked: switch bad from store or wiring failure or wiring wrong.

There are two stages to troubleshooting this.
First stage:
Pick any switch & label the lever positions with a '0' for up & a '1' for down. Leave the lever in the '1' position. Label it S1.

Flip one other switch [label it S2] until the light goes on & label the lever position with a '1'. The other lever position gets a '0'.

For the light [L] being on, a '1' is shown in the table below & so now you're on the first row below.

S1.S2.L
1.. 1.. 1
0.. 1.. 0
1.. 0.. 0
0.. 0.. 1

Then step thru each row from the top row on down.

If these two switches are working, the light should go out, then stay out & then come back on.

Report back on what happened with the light.

 

There is most likely a wire transposed on a terminal. The 4 way should have both wires from the same cable on the same color screws. What color and how many wires are at each switch? We need to know the color of the screws are. Harper has posted a diagram to help you understand.

Skip the testing above. It really does nothing to narrow down the issue.