The both solution are acceptable.
You can write a MT4 code outside and just attach the MT4 code to the platform. The system will convert your strategy automatically to a Java code when you will press Run button.
You can write a MT4 code in JForex. The system will convert your strategy automatically to a Java code when you will press Run (or Compile) button.
According to the question "what's the "MetaTrader 4 Beta version" said on your website?", please refer to any Dukascopy accounting manager:
email:
[email protected]phone: +41 (0) 22 799 4888
callback:
https://www.dukascopy.com/swiss/english/about/callback/