Much like a sport, instruments require endurance. You're exposing your body to unnatural positions and motions, so if you practice daily, you can build that up.
However, you say you only play every so often and up to 4 hour sessions. That is hard on your hands, which is why you have pain–you aren't used to it.
I suggest you practice at least 20 minutes per day, and during your play-times with other bands, ask for quick five minute breaks to rest your hands.

Remember–you have to take care of your hands just like the rest of your body! It's strange to think of your fingers having endurance, but it's true.
Outside of practicing, you could try ice to reduce swelling. Just hold the ice in your hands for a few minutes. You can also stretch out your fingers before and after playing.

If you don't have the ambition to practice, I suggest you stop playing. You'll only damage your hands further, be it through carpal tunnel or bursitis in your wrists. Secondly, without regular practice, you drag the rest of the band down because you are not up to the same skill and cognition level.

So….PRACTICE!
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FifthSon, your answers are senseless at times just like the Secondary Dominant stuff. Do your homework first.
This person plays bass and guitar and their is no UNNATURAL POSITION ABOUT THIS. Both instruments utilize the same fingering and position. What's unnatural about this ? You won't injure your fingers either. I practiced 18 hours a day and now 63 yrs old, and the fingers are the same as when I was 18. Look at Grappelli…He played at supersonic speed till 80 yrs old.
Lower the action, and use light gauge strings, half -round or round wound if possible. This should help trememdously.
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As you pointed out, the obvious best way to cure this is to play bass more often and build up callouses on your finger tips, however, there are a few things that might help you out.
I play bass, and I used to have a problem with my fingertips blistering during practises or at gigs. An old friend and long time pro bass player told me this tip.
I will take a bottle of super glue with me to practises and on each finger, I put a couple of drops on the tip and rub it against my thumb in a circular motion, trying to completely cover the finger tip. Make sure you don't stay in contact too long because if they stick they're a pain in the back side to get apart. The super glue gives your fingers a rough finish and hardens them up. You can even put multiple layers on for more protection. Once you've finished, the glue will wear off, or you can just hold them under hot water and use soap.
Sounds strange, but that's my tip. Works for me!
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