ITs not the type of engine that requires the pump usually its the placement of the fuel tank. If the fuel tank is far away from the engine and you need extra long fuel lines in an engine that is gravity fed a lot of times there isnt enough vaccume to draw the fuel into the carb at a sufficient rate to keep the engine running. The difference is for an example a YS engine is completly pressurized and the fuel is injected into the engine. Typically you need a fuel tank that can withstand 15 or so lbs of pressure without blowing up. Seeing as though there is a pump in the enigine you can now place the fuel tank on CG (good for 3D flyers) which is typically far away from the engines firewall. Other non pressurized engines need a pump added to it to help the fuel delivery to the carb so it doesnt starve. Then you can put the fuel tank where you want it. Sometimes where a pump makes the difference is when an engine is on its side or inverted and the middle of the tank os way above or below the carb. In that case you would still need a pump reguardless of 2 or 4 stroke type. My personnal issue with adding a pump to an engine is there is more components to worry about and the added weight. A lot of people dont like to use YS's because they are so heavy but by the time you add a pump onto an engine that was a few oz lighter than the YS you are so close to its weight it doesnt matter anymore and YS's really arent that difficult to mess with. So again its not the type of engine 4 stroke or 2 stroke its placement engine to fuel tank.