I am going through a similar furnace fan motor replacement problem right now.
The motor happens to be a Westinghouse, 5.6 inch diameter #48 frame, 3 speed, 115 volt, 1/5 HP, etc. It too has 4 wires, but only the black and white ones are used, the other two (green and red) having been taped off these last 35 years. This particular motor starts up and runs for maybe 10 seconds when very cold, 3 or 4 seconds when warm. I have verified that it is a motor problem by wiring it directly to 110 V, the plate says it is thermally protected and I suspect that it overheats internally and shuts itself down - bearings, windings, whatever, I don't much care, I just want heat and it owes me very little after 35 years (-:
FWIW, etc.
BTW, I don't know why they used a 3 speed motor and I havn't figured out how the 3 speeds are selected, perhaps by some switching between the wires that are used and the two that are not. I thought the air speed used to start low initially and then speed up as the furnace got up to temp, but I have become so used to this house that my memory isn't very clear on that.
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