Just use an Autonomy mod and you'll see that what The Sims 4 already has a chemistry much better than previous iterations, and yes, there definitely is a sort of system there, I'm confident of this because it used not to have one, either it's recent or maybe it was already there before but they tweaked it to show more. I noticed this because I've played with an Outgoing Sim and noticed that despite the trait and having a decent level in Charisma, there was still Sims who were hard to socialize to, and even with friendly interactions it resulted in Embarassed buffs.
The thing is, chemistry as it works in The Sims 2 and 3 does not make sense, that's not how it works in real life, you don't say "hi" to someone and then instantly know if that person is the love of your life. Also people have bad moments, in real life a Goofball could have Angry moments, it doesn't make sense that having a Goofball trait levies in Chemistry if the other Sim never sees the person actually behaving as a Goofball.
The Sims 4 is more about behaviour being defined not just by innate qualities but also by the moment, I like how Sims that otherwise could work together could just meet at the wrong place at the wrong time and because of bad moods they end up hating each other instead.
Now The Sims 2's Attraction though is a different deal, unlike Chemistry it would be for romance only and could represent, well, superficial attraction, so it would be tied to appearance, turn ons and turn offs (job, muscle/fat, clothes, etc), and maybe like The Sims 3 a random zoodiac sign to add some volatility to it. And BTW, Attraction is not a static thing since things that factor in it can change, like being turned on by muscled Sims and then that Sim becomes fat over time, could make for some very interesting situations like a couple slowly losing the "spark" over time.
