Almost all credit cards offer a grace period, where you don't owe any interest, as long as you paid the previous statement in full. See https://www.consumerfinance.gov/ask-cfpb/what-is-a-grace-period-for-a-credit-card-en-47/. So you would have at least 21 days (potentially significantly more, depending on where in the billing cycle you are) to then get the 401k loan and pay off the credit card. No credit card interest involved.