Sometimes the thrust washer gets lodged in the front end of the propeller.
If not,...Armed with the propeller shaft & the propeller, the thrust washer could be fabricated.
One of the thrust washer's jobs is to stop its forward edge & the propeller from being able to go forward too much and consequently rub on the gear case housing and retaining ring.
Its other job is to keep the front edge of the propeller's hub centered.
I have attached a picture of this item #49 from
Since you are only dealing with a 4 HP Merc that isn't supported much by Mercury Marine,...Rather than concerning yourself with the related taper on the propshaft,...IMHO,...Your machinist could fabricate a simple washer with its I.D. at a clearance measurement to the general prop shaft, and the O.D. at the measurement that would fit snug into the forward end of the prop's hub. The washer's thickness would create the separation between the front of the prop & the gear-case & retaining ring.
Of course, if your machinist looks at the taper, and can duplicate it for a reasonable fee,...You could spend that money.