user: Question: Which Surface has a Round of 4th Round, and a Player of Maria Sharapova?
Columns: ['Player', 'Ranking', 'Event', 'Surface', 'Round', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['2008 WTA Tour', '2008 WTA Tour', '2008 WTA Tour', '2008 WTA Tour', '2008 WTA Tour', '2008 WTA Tour']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Round" = '4th Round' AND "Player" = 'Maria Sharapova';