I`ve been playing guitar for nearly 4 years and can play pretty well; however, I just cannot seem to write any songs. I don`t know much about theory, would that help to learn?

Song writing theory? Yes absolutley. Buy a book as soon as possible. Musical theory not necessary. The song structure, both musical & lyrical will be covered in a half decent book, which must outline the different, "recipes", formats or formulas ... whatever. Good luck. Strat Man Dwight

Sure, try the book for about 10 or 20 bucks. They have a lot of instructions to do it right, and if you have an idea, its better if you know how to write it down properly and get it in the right structures.