This is so not a big deal. The last four should already be considered compromised given how often they appear on receipts, both physical and digital. Even with the last four known there's still plenty of entropy in the unknown digits. As long as there aren't any other massive fails on Google's part, you're safe. Also, most retailers store this info unencrypted and it doesn't violate any PCI DSS rules.
Source: http://codinginmysleep.com/2011/12/on-the-google-wallet-problem/
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This is so not a big deal. The last four should already be considered compromised given how often they appear on receipts, both physical and digital. Even with the last four known there's still plenty of entropy in the unknown digits. As long as there aren't any other massive fails on Google's part, you're safe. Also, most retailers store this info unencrypted and it doesn't violate any PCI DSS rules. Source: http://codinginmysleep.com/2011/12/on-the-google-wallet-problem/