Sorry to disagree with the other answerers, but the first six digits (not four) of a card number are known as the issuer identifier range. New ranges are FREQUENTLY added or removed to this list. You would typically need to be on a mailing list from the card issuers to even attempt to keep up to date
This document from Barclays (UK) for example shows a number of revisions in the last few months, such as the addition of a 6440-6599 range for Diners card. http://www.barclaycard.co.uk/business/documents/pdfs/bin_rules.pdf
Basically, attempting to identify card from the IIN is tricky at best. It's not clear why you want to identify a card from the IIN, but it typically isnt necessary if you are performing authorization through a payment gateway