Could you not use a flat mud ring and mud over the edges? https://www.homedepot.com/p/4-in-Square-Single-Device-Mud-Ring-Flat-787/100686866
If it doesn't recess enough to plaster over, mount your switch in one side and use a receptacle/blank combo cover plate.
If it doesn't recess enough to plaster over, mount your switch in one side and use a receptacle/blank combo cover plate.