MetaTrader 4 (MT4) is a trading platform developed by MetaQuotes in 2005. It is primarily associated with currency trading but can be used to trade a range of markets including indices, cryptocurrencies, and commodities via CFDs.
MT4 is extremely popular because it can be fully customized to your individual trading needs. It can also be used to automate your trading using algorithms that open and close your trades according to specific parameters.