Day 1: Borobudur, Mendut and Pawon (the temples are lies in Magelang, quiet far from Yogya)
Day 2: Taman Sari, Alun-alun and Kraton Jogja, Malioboro, Gudeg Mbarek Ibu Hajjah Amad (Yogya special tradisonal food), Kota Gede
Day 3: Prambanan and the others site (Candi Boko etc)
Day 4: Parang Tritis Beach
But 4 days is not enough. There are many places in Yogya that i didnt mention above.You just need more days to visit another places in Yogya.