let's say you're taking airport train both ways, it easily takes 2 hrs off your layover. another 1 hr for buffer before boarding time. 30 mins for terminal/train station changes.
there's still 3 hrs and 25 mins you can use, depending what you would like to.
the airport train station in Jakarta is Sudirman Baru / BNI City (same place)
near the station, not much to see. you need to navigate further for some some sightseeing
you can go to National monument, Istiqlal mosque, Jakarta cathedral for sightseeing (all near each other)
you can go to Plaza Indonesia / Grand Indonesia for shopping, eating etc, sightseeing around the Bundaran HI
... I don't know ...
thing with Jakarta is, if you knew exactly you want to do, it's always doable to do anything. if you don't, you'll end up confused in the traffic and thinking "what have I done?"