I am looking to use a piece of wood to lay under my sofa cushions because it sinks to much when you sit on it. I can't afford a new sofa so I was told I could use a piece of wood under the cushions. I need it to support a person 300 lbs- 400 lbs. They are disabled and do a lot of sitting.
3/4" plywood will do the job, but the outcome will depend on how the sofa's supporting structure is built. You'll want to cut the sheet to a suitable shape and bullnose the edges with a router or sander to prevent it from cutting through the fabric of the cushions and the base. It probably will eventually, though. You could also wrap it with old sheets or similar to protect the sofa.
As @isherwood has said, however, make sure your springs are not dangling. Easy way to do this is to flip the couch over and pull the bottom fabric off, usually stapled. Look inside as it should be hollow, and you will see the springs. On the front edge of the seat area, they are usually mounted to the wood rail. On the back they could be mounted to a rail or perhaps hung by a fabric strip. If the strip is torn and the spring has fallen, it needs replaced. A board won't support the weight if the fabric hung springs have all fallen. If the sofa has been sagging more over time, then the springs are just worn out, and a board could work to prop the cushions up. Again as long as the springs are still intact.