Trying to get some advice because I’m not really sure what to do at this point.

For some background, we have cats Uno, Dos, and Tres. Uno is around five years old, while Dos and Tres are a little under one. We already had Uno when we got the other two as kittens, and Uno acclimated to them fairly quickly. Some slight hissing and swatting for the first week or so, but since then they’ve been fine.

About 6~ months ago a stray started coming around, let’s call him Quatro. He’s super friendly and loves scritches, and recently moved into our back porch full-time, so we decided to bring him inside. It’s worth noting that while he was on our porch we’d occasionally crack the door and let all the cats interact, and while Dos and Tres would just sniff at Quatro, Uno would hiss if he got too close.

We put Quatro in a spare bathroom and started the acclimating process, and it’s where the weird behavior starts. Uno would sniff/eat at the door and have no issues. Sometimes we’d briefly put Quatro in a cabinet (we put a bed in there for him) and let Uno into the room to sniff around, and he’d have no issues. We could even let Uno look at Quatro from across the room and he’d have no issues. But as soon as Uno gets close to Quatro he starts hissing, swatting, and occasionally going for bites. They never actually fight (and Quatro never hisses/swats back), and it’s never an extended “engagement”, Uno just hisses/swats/bites then walks away. I put not acclimating “but in a weird way” in the title because of it; Uno never reaches the point of full-on aggression, but he’s clearly not happy.

My problem is this has been the status quo for a couple of weeks now. Uno will still hiss and swat at Quatro (but only when he gets close!), with the behavior not improving whatsoever. I know the usual advice is “start the acclimation process all over again”, but I feel like we’ve effectively done that multiple times and we’re not getting anywhere.