How to say: "I have been studying Chinese for 4 years."
Basically, all of your three sentences are correct, but different in emphasis. 

The sentence 1 "我已经学习中文四年了" is the best match for "I have been studying Chinese for 4 years." In this sentence, you just tell the fact and try not to emphasize anything.

The sentence 2 "我学习中文已经四年了" emphasizes "四年了". Once you said sentence 2, you would probably want to say more on the result of your learning. Like, 我学习中文已经四年了, 我现在可以用中文进行简单的日常沟通了(I can make simple communications in my daily life in Chinese)...blah, blah

The sentence 3 "我已经学了四年的中文" emphasizes "学了四年". Once you said sentence 3 我已经学了四年的中文, you would probably want to say more on the negative side (different from sentence 2, where you say more on the positive side). Something like, 我已经学了四年的中文, 但我还是说不好(but I can't speak it well) or 但我还是什么都不会(but I still know nothing)。