I believe they explained it nicely to you:
In order to use MT4 you want to open an MT4 account.
I am sure you may then internally transfer money from one account to another.
Then you can shut the first account in case you do not need it .
What is perplexing about it and can you not ask them if you do not understand (?) Something about the procedure?
Anyhow, you can go here and ask , a FXCM representative.