MetaTrader 4 is based on the MQL4 programming language, while MetaTrader 5 is
based on MQL5. Both languages can be used to develop custom indicators and expert advisors. However,
trading programmes developed through MQL4 are based on order system, while MQL5 uses a positional
system.
Compared to MQL4, the MQL5 language has more back-testing capabilities. MQL5 can
be used to simultaneously back-test multiple currency pairs. Traders can use the language to write and
alter scripts. However, many traders might find MQL4 easier to understand and use.