MT4, released in 2005, and its successor MT5 dominate retail FX because brokers licence the server side and clients use a familiar terminal everywhere. Most third-party tools, signal services and automated strategies are written for them.
Practical quirks matter. MT4 uses hedging accounts by default, allowing simultaneous long and short positions; MT5 introduced netting accounts, required for US brokers under the fifo-rule. Chart data comes from the broker, so histories differ between firms.
Example: a strategy backtested on one broker's MT4 history shows 1,240 trades; the same code on another broker's data shows 1,196, because the two feeds disagree on wicks by a pipette or two.
Related: expert-advisor, hedging-mode, position-netting, fifo-rule