Hey Rover People, I got an 2001 disco se w/250000+ miles. The coolant started pressurizing when hot, and the upper hose popped off of the radiator. I changed the hoses, thermostat, and inspected the water pump. It still happened so maybe head gaskets. I got the engine broken down and the number four piston is totally clean compared to the other pistons. I’m thinking I have a hole in the water jacket/piston wall/engine block. Any thoughts??

 

This sounds about right. Typically if there is a coolant leak in to the combustion space you will experience:
1) A pressurized cooling system
2) A "steam cleaned" piston

Now this can be caused by a faulty head gasket or a cracked block, here's hoping it's "just" the head gasket! Do you happen to have pictures of the heads/gaskets/piston? Some folks with a keener eye than me can sometimes spot a fault.

 

There is no water passage next to #4 so it could be a cracked block. Pictures would help but I would recommend to pressure test the block before going further.