Uhmmm no not exactly. Like Shankly said, tell us what you miss. "Four years ago" (Note! Write the number as a word, please) is just a time. "I miss how things were four years ago."

No it's not correct, writing "i miss 4 years ago" is stating that you miss the number 4, some years ago. You need to state what you actually miss.

Well, yes, it is. It means you miss the time you were having four years ago.

No, this is not complete, and does not make any sense. We don't know what you missed 4 years ago.

"I miss four years ago." Is grammatically correct (after capitalization and spelling the word.) The problem some are having with the sentence is it is not very informative. It causes more questions than answers. But grammatically, that is not an issue. The main question we have is "What was so great about 2008" or "What has happened that has made your life suck lately."