Hi,
A Mosfet has no stray voltage, it has leakage current
--> in your case max 500uA when very hot, usually less than 1uA with your measured 0V V_GS.
Your valve is specified with 24V and 4W, this means about 144 Ohms (170mA)
--> with 1uA of leakage current you may expect less than 1mV (70mV)
But your measured 4.5V means about 30mA
Your diode is 30V, 300mA rated. Double schottky diode...still unclear how you connected them.
30V is a bit low for 24V nominal voltage.
....waiting for the schematic and the pictures....
Klaus