Can coolant get into the combustion chamber?

Yes coolant can get into the combustion chamber. Under normal conditions it will not. However if the head gasket fails or on some vehicles if a intake gasket fails coolant can enter the combustion chamber. It sounds to me from what you described that the head gasket failed and pressure from the number four cylinder was able to enter the cooling system. That pressure may have over pressurized the cooling system causing the seam of radiator to fail.