The Samsung Galaxy S 4G was announced just a month before the Galaxy S2. While the former is just a rehash of the successful Galaxy S, the latter is a considerable leap. The first difference between the Samsung Galaxy S2 and Galaxy S 4G is the size of the screen. While the Galaxy S 4G retains the 4 inch screen of the Galaxy S, the Galaxy S II gains a slight increase to 4.3 inches. The overall size of the device did not increase as much and the Galaxy S2 is even thinner than the Galaxy S 4G.
Another difference between the Galaxy S II and the Galaxy S 4G is the processor used. The Galaxy S 4G uses the same 1Ghz processor from the Galaxy S. On the other hand, the Galaxy S2 uses a dual core processor that is running at a faster speed of 1.2Ghz. Android phones benefit greatly from having multiple cores because of multiple applications that run concurrently in the background. Apps can be organized on the two cores to balance the load evenly or to give more processing power to the one on the foreground.
The Galaxy S2 also has better specs when it comes to the cameras. The 5MP camera of the Galaxy S 4G is good enough for most people and it can record HD videos at 720p. The camera of the Galaxy S2 receives a nice bump to 8MP for even better photos; it can also record 1080p videos thanks to the much faster processor. The front facing camera, though not as important, is also improved to 2MP for clearer images during video calls.
Lastly, the Galaxy S2 is equipped with a lot more internal memory; it comes in 16GB and 32GB variants, while the Galaxy S 4G only comes with 1GB of internal memory. Storage is not really a problem for these phones because they have a microSD card slot that can accommodate up to 32GB. But it can affect the number of apps you can install on your phone. Most apps need space in internal memory; and unless you root your device, the 1GB internal memory of the Galaxy S 4G would run out pretty fast.
Summary:
The Galaxy S2 has a bigger screen than the Galaxy S 4G
The Galaxy S2 is much faster than the Galaxy S 4G
The Galaxy S2 has better cameras than the Galaxy S 4G
The Galaxy S2 has way more internal memory than the Galaxy S 4G