What is the 4-valve engine?

An engine has valves that let the air-fuel mixture into the combustion chamber to be burned and then draw out the exhaust gas after the combustion. A conventional engine has one intake valve to let in the air-fuel mixture and one exhaust valve to let out the exhaust gases. But the 4-valve engine has two intake and two exhaust valves. Just like having two doors lets more people enter and leave a room in a given amount of time than one door and more exits make for a more efficient flow of people, four valves is better than two. Four valves give an engine steadier low-speed performance and a better acceleration feeling. That’s why most race engines and high-performance engines have four valves. For example, Yamaha’s YZR-M1 MotoGP race machine has four valves.