It sounds like a hardware problem. CHDK software doesn't know anything about how many hubs or what kind of cables you have.
I would try to use a process of elimination to narrow down the problem. For example, if you just plug one camera into the extension does it work? If it does, then try extension -> hub -> one camera, then extension -> hub -> 4 camera etc.
There's a lot of marginal USB hardware out there, most manufacturers only care that it works in typical consumer use (if that) so it shouldn't be a surprise if it doesn't meet spec.
Are your hubs externally powered? They should be.
If you post the brand / mode of hubs that you are using, that might be helpful either to you or people trying to do the same thing in the future.
