Fiancé Rich Orosco's proposal was simple – and followed by a surprise Mexican fiesta party

Actress Julie Benz is taking on a new role: fiancée!

The former Dexter star, 39, is engaged to her boyfriend of four years, marketing executive Rich Orosco, her rep confirms to PEOPLE.

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Orosco, 38, popped the question with a customized Kwiat cushion-cut diamond and platinum engagement ring – complete with a diamond frame and milgrain edge band – on Sunday in the backyard of the home they share in Los Angeles.

“It was absolutely perfect,” Benz, who can next be seen on Royal Pains, tells PEOPLE.

Nearly 60 of their closest friends and family – and a Mariachi band! – helped them celebrate their engagement at a Mexican-style surprise party Orosco planned.

“The proposal was simple and beautiful – and then I was shocked by the pop-up surprise Mexican fiesta engagement party,” she says, adding, “What girl doesn’t want to be serenaded by a Mariachi band surrounded by close friends and family?”