If a set of obfs4 is working fine should they be kept indefinitely as long as they're still working? How would periodically getting a new set of bridges improve security? Potentially compromise security? Thank you.
Bridges are like guards, there are advantages to keeping them and advantages to changing them.
As a general rule you want to keep guards for as long as you can. Given a hypothetical pair of evil nodes in the network, a guard and an exit who together can deanonymize you, every time you pick a new guard you roll a dice. If the dice roll is unlucky, you pick the evil guard and lose. The less often you roll the dice the less likely you are to pick the evil guard. For more details on this, see Improving Tor's anonymity by changing guard parameters.
On the flipside, guards can track you. One of the problems Tails faces is that because it doesn't persist, it doesn't keep guards. This is problematic for the reasons mentioned before but also advantageous since a guard is actually a long-term identifier, which is something Tails is designed to avoid. This is even more pronounced in bridges, since typically bridges receive far fewer users than guards. Imagine some hypothetical Eve (a passive observer, who only looks but can't interfere) who can see your home internet connection connecting to some bridge address, they can't see the traffic but they can see you're connected (in the case of obfs4 this means you know the bridges public key). Now imagine later, you disconnect and Eve now sees a user connecting to the same address from a hotel in another city then some time later sees that connection stop and some time later your home connection is re-established. Eve might suspect that you briefly visited that hotel. This may not be something you would want Eve to suspect.
Outside of the scenario above, you should try to keep bridges for as long as possible. For the scenario above you may want to grab a distinct set of bridges for any trips you make to places where you may participate in sensitive activities, or just because you think Eve is a creep and should mind their own business.