I would suggest putting her in a playgroup not necessarily a class. A playgroup will allow for her to be herself and interact with English speakers.
Also, at home I suggest playing English music in the background while she is playing so she becomes familiar with the sounds of the language.
You may consider creating a spot (such as a chair or sofa) where you read English stories to her. She is 4yo so when you read the story just read the English try not to translate it - if you begin translating she may become dependent on always hearing the translation.
Once you have read the story to her a few times she will begin to make associations on her own.
You can also create one or two mornings per week where you speak English to her. If she doesn't understand it is ok just continue and use body language. You may repeat for or 5 or more times but that is ok.
If you are really keen on putting her in a class I would suggest YWCA or British Council. Also have heard that "Hooked on Phonics" which is US-based is coming or is in Hong Kong now.
