Qué dirán is a powerful Spanish phrase that literally means "what they will say." It's often used as a noun, el qué dirán, to refer to the fear of gossip or what other people might think of your actions.
In 'Felices Los 4', Maluma sings, "No importa el qué dirán, nos gusta así" (It doesn't matter what they'll say, we like it like this). He uses this phrase to express a rebellious attitude, dismissing social judgment about his unconventional relationship.
Maluma's "Felices los 4" is a super confident and catchy song about a very modern kind of relationship. He's talking to a lover who leaves as soon as the sun comes up: "A penas sale el sol y tú te vas corriendo" (As soon as the sun rises, you run off). He insists he's not hurt or jealous about her seeing other people. In fact, he's so sure she'll come back to him that he says, "No me importa un carajo porque sé que volverás" (I don't give a damn because I know you'll come back).
He doesn't just tolerate the situation, he actually proposes a solution. He sings, "Vamos a ser feliz, vamos a ser feliz, felices los cuatro" (We're going to be happy, we're going to be happy, the four of us happy). He suggests that he, his lover, her other partner, and maybe his own other partner can all be happy together in this open arrangement. He even jokes, "Te agrandamos el cuarto" (We'll make the room bigger for you), making it clear this isn't about traditional romance. It's a bold song about a relationship with no strings attached, where everyone involved agrees to the terms.