If you don't want a cushion that's hard as a rock or a mattress that sinks when you sit on it, you have to check the compression or ILD number. Compression means the number of pounds necessary to compress a piece of foam 25% and still bounce back to the original thickness. For example, a 4 thick piece of foam that requires 33 lbs.
To pack it down 1" and have it return to its full 4" thickness has a 33 lb. Do not open with a box cutter. Use a ball point pen or a pencil. Start by making a little incision and break it from there - durability : will last 7 to 10 years. Your item must be unused and in the same condition that you received it in.