I recommend the following dictionaries for English 4, 5 and 6:

Norstedt (translation from Swedish into English and English into Swedish).

Cambridge Dictionary (translation into English and other languages such as Arabic, Polish, Greek, Chinese etcetera).

You can access Norstedt and Cambridge Dictionary online or you can download the apps to iPad/iPhone/Android. A mobile app or online dictionary is a fast and effective way to look up new words.