Often just referred to as MT4, MetaTrader 4 is in many ways the classic and original trading platform for retail (non-institutional) forex trading. Although the platform was first released as early as 2005, it is still in use by many regulated forex brokers to this day.
One of the main reasons for the popularity of MetaTrader 4 is the huge selection of trading algorithms, known as Expert Advisors, that have been developed for the platform over the years. These trading strategies are available via the integrated marketplace inside the platform, where traders can pay to get access to them. MT4 is therefore also popular among trading strategy developers, who can use it to earn extra money from selling their trading algorithms.