Not entirely sure on the actual origins of this specific number but I think a crucial piece is that it passes the Luhn algorithm check [0] that is used to validate card numbers (and in generation of them to make single digit errors not result in another valid card number). 42 itself also passes the test but card numbers usually come in blocks of 4 digits so 4242 probably seems more card-like.
As to where the 42 from the Guide came from, I would recommend consulting The Hunting of the Snark.