Credits are automatically redeemed so you don't need to do anything. If you have $4 available at a specific location and don't use them all, the remaining balance will be left over to use on future transactions. Credits do not expire so you can use them anytime.
If you have Smooth Credits available from entering a promotional code and also have a Store Credit at a particular location, credits are used in the following order:
Store Credits --> Smooth Credits --> Payment Method Charged
To view the different types of credits click here.