In the early Renaissance the city of Florence became known for its arts and culture and also for its wealth. But it was a small republic and was constantly being threatened by surrounding city states. The fact that it seemed always to hold its own caused the city to take on the figure of David as its symbol. The depictions change throughout the Renaissance and into the Baroque periods...it's an interesting look at history through art!
The first 2 are by Donatello, the 3rd by Michelangelo, the 4th by Bernini.