I have enough room for a box 38w*15t*20d and the xcon recomends 4 cubic feet. whats the optimal size

Just because you have all that space doesn't mean you need to make the box that big. Just make one for 4cf like recommended.

Like said above, check out the suggested enclosure designs.

id maby do a 5 cuft box with lots of port area if you want to move allot of air

The manufacturer's recommended enclosure sizes are going to be optimal. Most give mininum and maximum, outside of that range is not going to improve anything. As I told you in the other thread, pick a sub and contact a box biulder or designer and tell them your goals.

The 4 cu ft will be optimal for sounding good. Smaller will get peakier sounding.

The next thing to realize is that a ported enclosure will include the volume of the port, sub and building material, so a 4cu ft box will be more like 5-5.5cu ft after port, sub, and building material volume is added to that 4cu ft internal volume.