The SOP is the SMD equivalent of the DIP component package.
SOP-4
The SOP-4 is an easy to solder, medium sized, 4-pin SMD package. Commonly used for small solid-state relays (Vishay makes quite a few of them). Related to SOP-4L, which is the same width, but longer.
Photos And 3D Models
A photo of the SOP-4 component package.
A 3D model of the SOP-4 component package. Image from www.digikey.com.