21Mbps is the theoretical maximum for hspa+ ni one will get it, but in strict laboratory conditions it's possible (sat next to the transmitter, ideal conditions and when the planets correctly align.
4g is just 4th generation mobile signals, 1g was the original used in the late 80s designed for calls only, no data, 2g is edge and gprs it can handle data but it's not ideal, 3g is 3g, hsdpa and hspa+ (some refer to the latter as 3.5g) which offers acceptable data speeds, it's the first standard to be designed around data and 4g is LTE and WiMax, WiMax is virtually obsolete already, a lot of US carriers have now moved from WiMax to LTE. 4g offers a theoretical maximum speed of 1Gbps but you will probably get 100Mbps and less.
Also the GT-I9300 does not support 4g, however of you have the GT-I9305 it will support 4g (check model number under about phone)
Sent from my GT-I9300 using Tapatalk 4 Beta just to annoy Gus
Device: Samsung Galaxy S3.
ROM: Whatever I feel like at any one time
Kernel: Please see above