1. 4 Stroke engine
Stroke is defined as the movement of the piston from the 'bottom dead center' (BDC) to the 'top dead center' (TDC) or the other way round, that is from TDC to BDC. A four stroke engine has 4 strokes. 
1º Intake.
2º Compression
3º Power
4º Exhaust.

2. 4 Stroke engine
1º Intake
The piston is at its highest point (TDC), the inlet valve opening and the exhaust valve closed.
The piston moves downward and it increases the volume between the valves and the piston, so the pressure inside the cylinder gets lower and lower. As the outside air pressure is higher, this forces the air-fuel mixture into the cylinder as the the piston is moving down
A moment later, the piston reaches the BDC ( bottom dead center) and the inlet valves closes.
A new Stroke is coming

3. 4 Stroke engine 2º Compression.
Both exhaust and intake valves stay closed as the piston move upward from the BDC (bottom dead center)
The piston moves upward, squeeze the fuel-air mixture into a smaller volume. The pressure gets higher and this causes the fuel-air mixture´s temperature to rise.
With high temperature, ignition is easier and combustion more complete

4. 4 Stroke engine Ignition and Power:
Just before the piston reach the TDC ( At the end of the compression stroke ) the spark plug fires.
The fuel-air mixture explodes forcing the piston down the cylinder.
The Power stroke drives the engine

5. 4 Stroke engine 4º Exhaust.
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. 
The piston now moves from BDC to TDC. The inlet valve is closed, the exhaust valve opens, and the piston, moving upward, pushes the leftover gases out of the cylinder.
BDC = bottom dead center
TDC = top dead center

6. 4 Stroke engine

7. 4 Stroke engine