I would recommend using it for wine. 4-10 is a pretty small space to try to workout in or do much else. If wine isn't your thing then I would recommend using it for storage. Both options are things that buyers like to see (more so the storage then wine room).
Lots of choices to make...We have a similar room in our basement that we use for tool and materials storage. It helps keep things out of sight and in one spot. You may also consider making it a small shop space of you need such a room. I used to have a darkroom that was about this size, but now I have a digital camera. I like the wine cellar idea, but I'm not that much of aficionado. If you're looking to maximize value, I'd just finish it out and call it storage. It will likely be the least expensive thing to do, and you never know what a future owner will see in the space.
Have you thought about removing the framing and opening it back up? No sense wasting square footage if you don't have a use in mind. I'm never afraid to back track in remodeling (if necessary, although I try my best to avoid making the wrong decision to begin with). After all, the purpose of the exercise is to get the space you want, not live with the constraints that were there in the first place.
Best of luck...
Keller Williams CCG