Whats purpose of four pole breaker?
The manufacturer decided that rather than develop a 200 ampere double pole breaker, they'd parallel 100 ampere breakers to create the breaker instead.
Each leg of the service is connected to a pair of 100 ampere single pole breakers in parallel. This configuration allows the current to be divided equally between the breakers, as long as it can be guaranteed that the resistance across each breaker is nearly identical.
Basically the manufacturer created a double pole 200 ampere breaker, using four 100 ampere breakers.
Before you get the idea to try this at home, 240.8 of the National Electrical Code (NEC) prohibits it.

National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 240 Overcurrent Protection
240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel