You can login with the other broker using MT4 (using your login/pass which you received from new broker).
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This is the general information about (it is for MT5, but same for MT4 for example)-
Find a Server by the Broker's Name:
https://www.mql5.com/en/forum/9950/page88#comment_3981953
and
https://www.mql5.com/en/forum/214820

How to Start with Metatrader 5
- 2017.01.05
- www.mql5.com
I decided to create this thread to help to myself and to the others to start with Metatrader 5...
in MT4 the list of brokers in not displayed.
how to find the broker XTB ?
sorry, I did it, thanks !
How can i registered a new broker in mt4 or mt5.Abybody can assist me here please
ChAudhary SAqlain:
How can i registered a new broker in mt4 or mt5.Abybody can assist me here please
This post #1 with links will help.
How can i registered a new broker in mt4 or mt5.Abybody can assist me here please

how to add new broker in MT4
- 2018.11.26
- www.mql5.com
Hi ! I am currently using MT4 with one broker. I have registered with another broker and want to use them both...

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Hi !
I am currently using MT4 with one broker.
I have registered with another broker and want to use them both.
How do I login / register this new broker in MT4?
Thanks !