Complex sentence: Then he studied at the Moscow School of Painting, Sculpture and Architecture for 4 years, attended the Saint Petersburg Imperial Academy of Arts from 1856 to 1860,which he graduated with the highest honours and a gold medal.
Split into simpler sentences: Then he studied at the Moscow School of Painting, Sculpture and Architecture for four years. After that, he attended the Saint Petersburg Imperial Academy of Arts from 1856 to 1860, and graduated with the highest honours and a gold medal.