Is 4 valves per cylinder more efficient than 2 valves per cylinder? Why?

4 valves is more efficient, it gets the mixture quicker in and the exhaust fumes quicker out of the cylinder.

because its stability and give high power 

In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power

Most engines have2 valves per cylinder,1 for exhaust and1 for intake of air and fuel. Todays more modern and more efficient engines utilize4 valves per cylinder to get maximum power and fuel economy. Again,2 intake and2 exhaust valves per cylinder. 

good luck....

Valves just increases the efficiency of combustion and shorten the lead time,but valves are also depends upon the types of piston ,type of engine like high speed or for heavy load condition etc. various other reason are also there for using4 valves 

A multi-valve engine has better breathing and may be able to operate at higher  (RPM) than a two-valve engine, delivering more power.

I agree with the expert answers.

FOUR VALVES IN A CYLINDER BETTER THAN TWO BECAUSE IT IMPROVES THE VOLUMETRIC EFFICIENCY AND AIDS IN SCARVENGING THE BURN''T GASES OUT OF THE CYLINDER .

4 valve vs2 valve. The name of the game is velocity and turbulence/mixing of the intake charge at differing engine speeds. At low engine speeds, one intake valve gives increased velocity, hence better gas mixing and better cylinder filling. If you open a second intake valve at low engine speed, the velocity drops dramitically, leaving poor intake filling and a lean intake charge. The result is engine knock and less torque.

On the other hand, at high RPM, breathing is the name of the game. The valves are open such a short length of time you need the maximum available intake area. Therefore2 intake valves work better at high RPM. All of this is of course subject to the exhaust system design.

A proper extractor exhaust can make a significant difference on a two valve system and a restrictive exhaust can nullify all the gains of a4 valve system. These are SYSTEM designs we are talking about, both intake and exhaust are related. Change one parameter and you must change the rest of the system or you have wasted your time. To wit, most guys who add street superchargers don't see much need to have4 valve heads because the supercharger is packing the cylinders as full as they can get at streetable RPM.

The real world test is to compare the RPM vs Horsepower (or torque)charts that come from dyno testing. Look at similar engine displacements with2 valve and4 valve heads. Then look at the curves for two entirely different engines with the same horsepower ratings.

For example, compare a turbo,4 valve,2 liter racing engine (500 Hp @10,000 RPM) to a naturally aspirated2 valve427 cubic inch street engine (500 Hp @5000 RPM). Now you see the complexity of comparisons! There are no universal answers, just better or worse application.