Explanation
The MQL4 reference says it is a built-in language for trading strategies, developed by MetaQuotes. With it a user can write Expert Advisors that send orders to the trade server, custom indicators that cannot trade, scripts that run once from OnStart, and libraries. Code is written in MetaEditor. MetaQuotes' MetaTrader 4 automation page lists the same pieces: the terminal, MQL4, MetaEditor and the strategy tester. MQL4 programs do not run on MetaTrader 5. That platform uses MQL5.
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Sources and last check
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