Are you really sure it is 100% legal to make a service based on MT4?
It may bring problems for the website who hosts the service. A broker who provides a MT4 service to it's users has to pay about 100K dollars..
From my understanding of this, brokers have to pay for the right to use the server that the clients talk to. But the clients are free of charge. I'm not sure, but how can be illegal to use their own API to save some data to a file and open it like a MT4 document for example? I'm not a lawyer, and maybe my theories are not that great!