What would happen if I have a 4 cycle engine with oil and gas mix?

I don't think the oil/gas mix will harm the engine, but the engine will probably emit greyish-blue smoke until the oil/gas is burnt off. Also, the spark plug will probably foul due to the oil, and cause the engine to burn rich and emit black smoke. The engine will probably run rough until all the oil/gas is burnt and you will have to remove and clean the spark plug often. Save yourself the "headache" and drain the oil/gas from the fuel tank.