Ok, so again, what 4MP cams did you upgrade from?

The 6MP and 8MP cams mentioned are still fairly "value" oriented, so other than the slightly larger sensor size of the 8MP cam, the specs on the 4MP cams that were replaced have got to be pretty crummy in comparison to make as dramatic of a difference as you are describing.

More than likely, the 4MP cams you'll reference use the same exact sensor as the 6MP that replaced them. You'll end up with a higher resolution/wider field of view for sure, but you'll actually get worse low light performance out of the 6s for obvious reasons. Other conditions should provide fairly equal results though.