MT4 is classic and best one.BUT MT4 allows only hedging. MT5, however, permits both hedging and netting. An example of netting is if a trader were to enter into a long position for 1 lot on say the EUR/USD and then enter a second lot in the same market, the initial order entry would simply become one position, as opposed to two separate positions.
MT5 is faster and more efficient than MT4. MT5 is a 64-bit, multi-threaded platform, whilst MT4 is a 32-bit, mono-threaded platform.