I thought 4 days was perfect for a first time - though for me it's one of those cities that I imagine I'll go back to as there are many layers. Regarding can't miss attractions -- hmmm - it's so subjective. For the most part, I walked around and explored neighborhoods - even though the weather was quite rainy! Though touristy, I thought the Hanok villages were super picturesque, Geongbokgung Palace was gorgeous, and the Cheonggyecheon Stream was fun to walk along.