How far back can I backtest on MT4?

The MT4 terminal allows clients to backtest their strategies via the use of the MQL4 language, and/or other proprietary MT4 plugins. The amount of historical data available for backtesting is dependent on the candle resolution selected. This data is automatically downloaded from OANDA's MT4 servers when a chart is opened on the MT4 terminal.

In addition to the historical candle data that's stored on OANDA's MT4 servers, the MT4 terminal builds new candles for opened charts based on the real time price stream it receives. These candles are cached on the client's MT4 terminal.