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I'm not at all familiar with XOOPS 1.x but that may be a possibility. Some don't like it, some do.
Also check out opensourcecms, I'm sure there are several there which support 4.0.6.
Xoops did support 4.0.6 for a while, even the 2.x series (I think). I'm sure you are not interested in this, but I'm thinking it would be too difficult to convert it so it works... not too difficult, but a **LOT** of work. Basically you'd have to convert any $_GET to $HTTP_GET_VARS, etc. and come up with replacements for a few functions that exist in php 4.1+ but not in 4.0.6.
I can understand not wanting to support certain versions of PHP as some were very buggy. I think the latest release 4.3.4 is supposed to be quite good though... maybe they can reconsider??