I am in the process of installing conduit and wiring for a spa. The approximate distance will be 150 feet from breaker to spa. I will locate the GFI/service/emergancy disconnect 10 feet from the spa itself. At the base of the spa will be a duplex junction box for the hard wire connections via a flexible waterproof whip into and through the spa skirt. A 50 amp breaker provides protection at the beginning of the run. The wire size I believe should be #4 (If this can be reduced without compromising performance or safety that would be great).
My question is:
Is it necessary to continue with #4 conductors for the last 10-15 feet after the GFI/service /emergancy disconnect ? If these can be reduced what would their size be?

For this size load and length, I would stay with #4 copper. The motor starting load concerns me and would affect the voltage drop.

The last few feet can be smaller #6 copper wire.

You may want to compare price of #4 copper to #2 aluminum wire. Conduit size needs to be considered. If you size conduit for #4 at 150 feet, you should be using 1-1/4",which will be fine for 3-#2 XHHW and 1-#6 copper for equipment ground. Just need to treat the aluminum connections with anti-oxidant compound, and torque properly as with copper connections.