The four language skills of reading, writing, speaking, and listening are highly interconnected and, therefore, cannot be taught independently. 

Listening and reading both rely on input from an outside source and require knowledge of the language, background knowledge, and comprehension skills.

The productive skills of writing and speaking are more complex as they necessitate taking knowledge of a language a step further to actually produce new language.  

All four skills are needed to successfully assimilate into an environment where the TL is used: one cannot simply listen without responding.  The foreign language classroom should mirror (as closely as possible) an authentic environment where the language is used.  Therefore, these four skills should be integrated into the curriculum.

If you are talking about reading, writing, speaking and doing, I think it's pretty self-explanatory. These four skills are essential to literacy--both academic and cultural.  Students need to know how to present themselves well in writing and in person in order to compete in our global society/economy.  They are crucial in the classroom because they are skills that not only apply to academic success and college readiness, but also to workforce success and readiness.