AFAIK, the only difference between the two is the ability to adjust the relative level of bass output on the 5000. If you look at the Ohm web site, you will see that both models are represented by a single speaker in the photo. The cubic footage application chart on the web site has the same maximum room size for both speakers, but the 5000 can be used in a much smaller room than the 4000. I believe that is due to the the bass output being overwhelming for small rooms without the ability to adjust it.
I would say that if you are worried that your room is now, or will someday be, too small for the lower size limit of the 4000, it is worthwhile to get the 5000s. Otherwise, save the money and get the 4000s. Plus, Ohm will probably help you upgrade from the 4000s to the 5000s if you ever need to, with a generous trade in allowance.
And John Strohbeen at Ohm is very accessable and easy to chat with. Don't hesitate to call him and ask which he thinks would be better for you.
