We have moved to another country when my child was 4. She was already bilingual, she speaks Dutch and German. Now she is going to an English school and she has stopped speaking either Dutch or German, instead she has started to speak English. Is this normal?
This is normal.
If you want her to keep up her Dutch or German, you'll need to provide opportunities to interact with people who speak Dutch or German but who don't understand English very well. This doesn't need to be immediate; once she is a native speaker of the language, exposure later in life - such as for example if you were to move to Germany in ten years, when she's a teenager - would work fine.