200 amp service implies 100 amps per 120-volt leg or 200 amps total. A 240 volt load would draw an equal amount from both legs, e.g., 30 amps, leaving 70 amps in each leg for other loads.
Just because a panel may be rated for 1000 amps doesn't mean that you have 1000 amp service provided by the utility. You're ultimately limited by the size of the transformer they supply and you have to derate accordingly if that transformer is being shared by other households.
That is a bizarre main breaker. Main service interrupts are usually a single 240-volt breaker or a ganged dual 120-volt pair. By "common trip", do you mean that all four are mechanically linked so that if one trips, they all trip?
Dean