Ironically I think after this strip was when people began wanting to make Peanut/Tarot/Max/Grape a quad couple because they all worked so well together. The final panel probably gave a lot of people hope for that. Of course back then they were probably on the lookout for Tarot and Grape after this one.
I don't think this guest strip is as old as you think it is. This guest strip is from 2021, in the middle of the Heckraisers arc, from the guest week right after Rick posted that at the end of the arc he was going to take a hiatus. It's probably the most recent strip containing all four of them.
Quotidie apud comicos novas fascias deprimo, quotidieque tristis sum.
They more or less are a quad. Grape's dating Peanut and Max, and Peanut's dating Grape and Tarot. This much is canon. Tarot doesn't have to date Max to make them a quad.
Quotidie apud comicos novas fascias deprimo, quotidieque tristis sum.
By canon I take it you mean you canon because the way the comic treats Peanut and Grape, they are just really close siblings and not dating.
Also, a quad is a romantic couple consisting of four people all dating each other. Peanut dating Tarot and Grape and Grape dating Peanut and Max makes Tarot/Peanut/Grape and Peanut/Grape/Max two throuples. In order for it to be a quad, the four of them ALL have to date each other romantically. (ie. Peanut dates Grape, Tarot and Max; Grape dates Peanut, Tarot and Max; Max dates Peanut, Grape and Tarot; Tarot dates Peanut, Grape and Max).
Peanut and Grape DO go on dates together, in canon, starting right after the first time they puppysat for King and Bailey. It's heavily implied they used the money to go on a date, with Peanut explicitly saying "you wanna go on a date?" And Grape saying "Eh, sure."
Quotidie apud comicos novas fascias deprimo, quotidieque tristis sum.
I think we were meant to see that as just two siblings going out together. But I know many people interpret things differently and I hadn't thought about them as a possible couple in a while even when I saw that strip for the first time.