No, it's not correct. There are two ways you could fix this. You can use the formal version of a prepositional phrase by switching the order of "for" and "what" and substituting "which" instead of "what." The ring for which I waited for 4 years. This feels very formal and stiff, though, especially for conversations or casual writing. The second fix is to use an adjective clause with the preposition at the end. There are certain words we use to introduce adjective clauses. "What" is not one of those words. You can find a list here: http://www.chompchomp.com/terms/adjectiveclause.htm I would recommend using "that" or "which." Or you can leave out the introduction word altogether. The ring that I waited for for 4 years. The ring which I waited for for 4 years. The ring I waited for for 4 years. (It may seem strange to have "for for" in a sentence, but it is very normal. You just need to realize that the first "for" goes with "that I waited for" and the second one goes with "for 4 years."

Sarah is absolutely correct, but I wanted to add, that you can switch it to the active voice and say: "I waited for the ring for 4 years." It depends on whether "I", "the ring" or "4 years" is the focus of the sentence. The ring that I waited for for 4 years. -> "the ring" is the focus I waited for the ring for 4 years. -> "I" am the focus. For 4 years, I waited for the ring. -> "4 years" is the focus.

"Is this grammatically correct?" = question "This is grammatically correct." = sentence