Since the service update on XM, newly offering MetaTrader5 Trading Platform, the XM Web-Trader is available only for MT5 traders.
In case you have chosen to trade through MetaTrader4 platform, you cannot access to the Web-based Trading Platform, but you need to have a MT5 trading account to access to the Web-based Trading Platform.
Other than the web-based trading platform, both MT4 and MT5 are available for Windows PC, MAC PC, Android devices and iOS devices.
You can find the download links for these platforms in the official website of XM.
Although both MT4 and MT5 are available for all account types, there are several different conditions between them.
Such as the Maximum Leverage and available Financial Instruments.
There are also differences of Specs between MT4 and MT5. You can find more details from the below article.