1998 Lexus GS400 ( code P0768 )

I own a 1998 Lexus GS400. For about a couple weeks now my engine light has come on and off. Recently this week it has stayed on full time when the car is on. I took the car to my mechanic who does all the maintenace work (oil change, brakes. Etc) He checked the light, he told me he got the code P.0768 (which is shift solenoid D in the valve assembly or valve 4). The car runs fine, no stalls & shifts seamlessly. I want to replace the shift solenoid D but don't know which one it is in the assembly. My question is; Which is Valve 4 or Solenoid "D"?