It's really a misnomer perpetuated by the fuel mfr's to call a fuel "two-stroke" or "four-stroke". The label is meaningless.

What matters most to the engine is oil content and type.
Nitro content is, of course, important, too. But the oil content and type is the major fuel factor which influences engine life.

Many 4-stroke mfr's recommend all-synthetic while some say add a little castor. Too much castor will lead to carbon deposits in the valve area which will impair the engine. A small amount of castor will protect the engine better during an overheated condition.

Check with the mfr for oil content and type recommendations and ignore the "4-stroke" and "2-stroke" labels on the bottle.