As others have stated, the wood/foam/wood floor is not 4" thick. My floor is about 2" thick. If it broke through and then feels solid, you may be hitting the top of a freshwater/grey/black tank. It sounds like the floor broke all the way through and the bottom fell until it hit something solid under the floor.
If you can't go from the bottom this is what I would do:
1) Cut out the bad area of the floor completely.
2) Take a piece of 1/2" thick exterior grade plywood with the width slightly less than the hole width, the length about a foot longer than the hole.
3) Put a long screw in the middle of the plywood that sticks up and acts as a handle
4) The plywood can now be slid into the hole one end through first using the screw to hold it and slide it. The plywood can be centered in the hole, overlapping the hole by 6 inches at either end from below.
5) Before putting the plywood into the hole, look at the bottom surface of the floor where the plywood would overlap it by 6". if there is some plastic sheet attached, remove it in that area until you get to wood.
6) Get some high-strength epoxy and apply it to the plywood piece where it overlaps the hole.
7) Put the plywood piece into the hole and center it. Pull up on the piece and rig up something to hold the piece against the bottom of the hole (by pulling on the screw "handle"). Let the epoxy cure completely (usually 24hrs).
8) You now have a solid base to build on. Fill the hole with whatever you like. I would use a piece of foamboard topped with a piece of 3/8" or 1/2" plywood flush with the floor.
Steve
