Hi Nao They're all correct. With 1 and 2 you can use either - they both have the same meaning. You don't usually have to say 'playing' because it is taken for granted in conversation. If you wanted it to be something else, you would say it, such as 'I'm good at watching tennis'. With 3 and 4 they can have different meanings, depending on what you are trying to say. With tennis, you can fairly safely assume that someone is 'playing' tennis. With 'English' you cannot safely assume that they are 'speaking'; there are many things that you can do, like 'study', 'read', 'write' as well as speaking. So, 3 is explicitly saying that you are good at speaking English, but 4 is not. You might use 4 in response to a question like 'What lessons do you prefer at school?' On the other hand, if a question directs your answer, you could use it like this: 'What languages can you speak?' - then your answer can be 'I'm good at English'.

I've answered at the same time with DanO, so it is obvious that both are correct :))

To be good at smth - to be able to do smth well. I think both versions are acceptable, let's wait what native speakers will say.