A four-stroke is a shorthand description of a four cycle internal combustion engine.
Four stroke engines do not require the addition of a thin oil added to the fuel for lubrication of the pistons. Adding oil to the gas is something one does with a two-stroke engine. That's why it is important, when borrorwing a motorcycle from a friend, to ask which type of engine it is, because if you forget the oil in a two-stroke, you're buying a new upper end for the engine.
The four stroke lubricates the piston using the internal crankcase oil. That's why there is a minimum and maximum oil hashmark on the dipstick. Too low, you're seizing the engine. Too high, and the bottom portion of the piston is hitting the oil at a high rate of speed, damaging the engine.
Another difference is the four stroke has valves and goes through four cycles:
- Down stroke after the spark plug fires (the power portion)
- Upstroke of the piston, which pushed the exhause out the valves
- Downstroke that sucks in fuel and air
- Upstroke that compresses the gasses, ready to fire.
Conversely, two cycle has the downstroke from the firing and the upstroke that compresses the gasses that get sucked in at the bottom of the down stroke.