Trato translates to "deal" or "agreement". While it's a common word, Maluma uses it here to describe a very unconventional romantic arrangement.
He sings, "Yo te acepto el trato" (I accept the deal), laying out the terms of an open relationship where both partners are free to be with other people. This word is the key to understanding the song's provocative theme of being "Felices los 4" (Happy the 4 of us).
Colombian superstar Maluma teams up with salsa royalty Marc Anthony for this high-energy, brass-heavy version of "Felices Los 4", and the story is refreshingly shameless. Right from the start, a lover sneaks away at dawn: "Apenas sale el sol y tú te vas corriendo" ("As soon as the sun rises, you run off"). Most pop songs would cue the tears right there. Not this one. Instead of getting jealous, the singer shrugs it off with "no me importa un carajo porque sé que volverás" ("I don't give a damn because I know you'll come back"). He does not want a ring, rules, or commitments ("no creo en contratos" ["I don't believe in contracts"]). As far as he is concerned, if she spends her time with someone else, nobody needs to fight about it.
The real punchline is the chorus: "Y si con otro pasas el rato / Vamos a ser feliz... Felices los 4" ("And if you spend time with another guy, we're going to be happy... happy, the four of us"). His math implies that both partners have someone else on the side, and nobody has to feel guilty. He even jokes about making room for everyone with "te agrandamos el cuarto" ("we'll make the bedroom bigger for you"). When the horns kick in and Marc shouts "Ataca Sergio" ("Attack, Sergio"—calling out his bandleader Sergio George), the whole song turns into a dancefloor agreement to ignore outside gossip ("no importa el qué dirán" ["who cares what people say"]) and keep the arrangement strictly fun.