The D-max 4.2 is only marketing. Sorry to say, Canon has try to avoid it, but the marketing department of Nikon and other want it. At the Swedish Canon site they wrote this, in translation,
"The dynamic range from CCD is 3.5 and the A/D conversion value is 4.2".

Theoretical, a 14 bit scanner can make a maximum of 4.2 but i doubt if it exist any CCD in the part of the milky way which can produce a d-max such hight.

So the marketing d-max 4.2 is a rewriting of 42 bit a/d conversion.

This is a nice article about that: http://www.scantips.com/basics14.html The link is from hamrick's site.