Clients can use Web Client and MultiCharts at the same time. The account is the same, and clients can use either platforms. However, the MT4 Exchange is a different account, and clients will not be able to use MT4 Exchange inter-changeably as they can with Web Client and Multicharts.
MT4 is a standalone third party trading platform which operates independently of your LMAX Exchange web trading account. Trades done on your MT4 account will not be reflected on your LMAX Exchange web trading account and vice versa.