4 strokers are typically more fuel efficient than 2 strokers because of the way the engines operate. To understand the difference, one needs to acknowledge what happens inside any combustion engine. There are 4 phases that can be labeled as suck, squeeze, bang, blow. In a 4 stroke engine, these 4 phases happen in sequence, one phase with each stroke of the piston. On the first down stroke of the piston, the intake valve opens and the fuel-air mix is drawn into the cylinder. Then, the valve closes, and as the piston comes back to the top of the cylinder, the mixture is squeezed. The spark plug (or in our glow engines, the glow plug) ignites this mixture causing a rapid expansion of the gasses, forcing the piston back down and turning the crankshaft which turns the propellor. On the return of the piston to the top of the cylinder, the exhaust valve opens and the spent gasses are expelled so the entire sequence can begin again.
In a 2 stroke engine, these phases overlap each other and the mixture is ignited each time the piston nears the top of its stroke. Because a 2 stroke engine uses ports in the sides of the cylinder for gas exchange rather than valves, some of the intake mixture is expelled with the spent gasses out the exhaust port and some of the spent gas remains in the cylinder for the next cycle. Because they fire with every revolution instead of every other revolution, 2 stroke engines are, generally speaking, more powerful than a 4 stroke of similar size, but also less efficient. This is also why a 4 stroke sounds so different than a 2 stroke.
I hope this brief explanation answers your question.