我来到中国四年了，但是现在我还不大会说中文。 You must say the sentence like that.“哪“ you used wrong. ”不可能“ is wrong either. Because you have been China four years.It impossible that you can not say any word. And accross the sentence you give ,we know you can say Chinese-_-

1.“哪”多数用来提问，比如 哪里——问句中等于when 哪个——问句中等于which one 在这里用“呢”更好些，不用也可以。it goes like“但是呢，” 2.“不可能”＝impossible do you mean that ‘my chinese is not good still?’ or ‘i can't speak fluent chinese?’ if you say yes, then you can't use “不可能”。“不可能说”actually means totally can't speak. “说不好”means can't speak good chinese or can't communicate freely. so I would say : 我来中国四年了，但是呢，现在我还说不好中文。 or 我到中国四年了，但是呢，现在我还说不好中文。 “呢”and the second“我”can be cancelled.

It would be better if you say in this way :" 我到中国四年了,可现在还是说不好中文." Do you really mean it? Then you'd better hurry up cause there are a lot of italki Chinese members are willing to help.

汉语的确是个比较难学的语言，如果可以的话我想我们可以exchange。 我想你想表达的意思是：“我来到中国已经四年了，可我现在还是说不好中国话”

多交流，即使你不会写中文，也能说普通话了