That's not what that image means at all. If you look closely, you'll even see 3 additional colors, plus white, from the 4 I'm guessing you identified.

Those are ERCOT load zones, a distinct concept and all within the ERCOT interconnection (grid).

On the markets side, Texas is made up of ERCOT, and then has portions in (descending order) MISO, SPP, and the non-market West.

In terms of "grids" Texas is mostly ERCOT, and then the Eastern Interconnection with a small smidge of Western Interconnection in the far west in El Paso Electric's territory.