I was going to pose same question as @Chrysso
I just had a quick look at the manual for the iConnectivity Audio4c (https://iconnectivitysupport.zendesk.com/hc/en-us/article_attachments/28155196501531) and if you don't have it in the correct Routing Mode then it sounds like it will just mix the inputs down to a stereo pair. Looks like you need to be in 'Play USB 1' (assuming you are using Port 1) mode. But that is based on a 30 sec skim of the manual so I could be wrong! Someone else in the community using this interface might be able to confirm.
From what you described, your ST track -> bus -> output routing all sounds fine.
When you say "BUT the stereo track is playing in both 1-2 and 3-4" do you mean on the Audio4c interface or in the ST mixer? Again, making assumptions, but given you can't route a track to multiple buses in ST, I'm assuming you mean in the Audio 4c.
Do please clarify things if our assumptions are wrong.