What type of power connector is this?

Looks like an adapter to power a 3½" floppy drive. The original power supply connectors for 5¼" floppies, hard drives, CD-ROM's, etc. were the common "4-pin Molex". When 3½" floppies were introduced they had a smaller power supply connector, and most PSU makers started to include connectors on cables that plug into those. That way you had both 4-pin Molex and the newer smaller power supply connectors available, often on the same set of 4 wires, to supply power to either type of drive.

But another option, especially if you had an older PSU with no such connectors, would be to supply an adapter like you have. It would have a 4-pin male Molex connector on one end (just like the "socket" on the back of a hard disk), and a smaller 4-pin female connector on the other end to fit into the "socket" on the back of a 3½" drive. The electrical power on the 4 wires is the same - only the connector is different.