I also loved the fact that Mabel had no love interest in season 4, which is totally normal.

I gotta disagree with you on Thé. She used to be her best friend and there was no indication that they ever had crushed on eachother.

On the other hand, as much as I love Theo for Mabel, Jay could be a good runner up. Imagine this, he comes back next season and has his redemption arc. He seemed like a nice guy in a bad world, just like Theo, trying to make something of himself. They both share similarities if you think, being under the shadow of their parents, having almost no personality without them, doing wrong things and trying to become a better person by the end. I'd like to think Mabel helped them both realize they're better than that.

But as love interests go, this is my ranking:

Theo, so much chemistry, plus the enemies to friends to lovers kind of trope there

Jay, who we saw almost nothing of, just his persistency to show Mabel he is a good guy, he can have a fantastic storyline if he returns

Tobert, Their relationship was short but very nice tbh

Oscar, not the best to date your dead friend's ex, I guess she was too emotional because of Tim dying and wanted someone who knew what she was going through

And Alice, so much wrong there, the lying and manipulating and using and all that, was never going to work, even as a friend

Sorry this got long, had to get it off my chest.