We have a 4 hole stainless steel (or something) kitchen sink: from left to right it goes hot water, faucet, cold water, and sprayer. Recently the sprayer crapped out on us, and my wife has said she would prefer something that doesn't have a hot and cold handle and instead is one handle that you adjust one way or the other for hot and cold. I have seen some good options for ones that include the detachable faucet as the sprayer that would work for us, and the fixture fits 3 holes. That would leave us with one unaccounted for. My wife doesn't want to get a separate soap dispenser, so is there some sort of plug or something that I could put in the 4th hole? No idea if something like that exists or not.

Yes. You can get a plug/cap to fill the hole. They have an assortment at HD/Lowe's or you can get one on amazon

https://www.homedepot.com/p/1-3-4-in-Sink-Hole-Cover-in-Polished-Chrome-K-8830-CP/100019753?cm_mmc=Shopping%7CG%7CBase%7CAll-Products%7CAll%7CAll%7CPLA%7C71700000014585962%7C58700001236285396%7C92700010802552406&gclid=EAIaIQobChMI2szsk6Sf3QIVgx9pCh3mXgEKEA0YCCABEgJQn_D_BwE&gclsrc=aw.ds&dclid=CPTF052kn90CFQW5TwodUfQCPQ

Perfect. Exactly what I needed, I just didn't even know what to search for on the website. Much appreciated.

I had same issue recently. Only other advise is I like the plug that hs the bolt and nut to tighten down as opposed to the tines that you bend back. The latter doesn't hold its postion as well and is a PIA toninstall

I like to put hand soap in the the dispenser so i don't have to keep a pump bottle next to the sink.

/I know she doesn't want this.....

Yeah I'm considering just putting that in and not using it. Might make it more appealing when we sell and she doesn't HAVE to use it. She just likes the prefoam stuff

Or put dish soap in it so you can just squirt it into the sink for hand washing dishes.

I put an all in one sink hose spray with a high neck and love it. In the fourth hole I put a drinking faucet and under counter reverse osmosis alkaline filter system.

I installed an air switch for the disposal in the 4th spot and its great. The wall switch was too long a reach away, so its nice having a button right there to kick on the disposal.