They will only use a license if they are setup to be User "Phones". If you configured four ports to be user phones, then it would use four licenses. It all depends on how many ports are configured as user phones. Here is a snippet from the "FXS port role" help section.
User phone. Allows advanced features to be configured on the phone such as voice mail. The ports are SCCP controlled and occupy a user license. Available features are further configured from Configure > Telephony > Users and Extensions > User and Phones > User Extensions.
Common area phone. A common area phone is typically an analog phone located in a lobby or breakroom. Advanced features such as call forwarding and voice mail are not available on these phones. FXS ports assigned to this role are configured on the Analog Extensions tab in the Users and Phones window (Configure > Telephony > Users and Extensions > User and Phones). No License required
Fax. Allows integration with features such as SIP Trunk or T.37 Fax to Mail since additional/special configuration is required for them to properly handle fax machines. (Configure > Telephony > Users and Extensions > User and Phones). No License required
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