A module with its part number starting with SK means it’s a Starter Kit.

A Starter Kit is a display module with a set of accessory parts, which are designed to aid the starting out of development with the 4D Systems product in question.

For example, a SK-GEN4-43DCT is a Starter Kit for the gen4-uLCD-43DCT display module and includes extra parts such as a 4D-UPA Programmer and a microSD Card.