MQL4 vs MQL5: What's the difference?

Different platforms entirely

MQL4 runs in MetaTrader 4; MQL5 runs in MetaTrader 5. They're separate applications, and code written for one doesn't run in the other without being rewritten.

Position handling

MT4 is hedging-only — you can hold multiple opposing positions on the same symbol. MT5 defaults to netting (positions on the same symbol merge into one), with hedging available as an account-level setting depending on the broker.

Language structure

MQL5 is built around a fuller object-oriented model with a standard class library (CTrade for order execution, CExpert for strategy scaffolding, and more). Older MQL4 code tends to be more procedural, though MQL4 has adopted much of MQL5's syntax over time.

Indicator access

MQL5 uses indicator handles plus CopyBuffer() to read values — an extra setup step compared to MQL4's older direct-call style, but it scales better for multi-symbol, multi-timeframe strategies.

Timeframes and testing

MT5 supports more built-in timeframes and a more capable Strategy Tester, including multi-currency backtesting in one pass — useful if a strategy trades several pairs at once.