It is possible to combine several exposures if you want light trails etc. There is nothing stopping you combining them in post. Stack the images in Photoshop or GIMP (free) as seperate layers and set the blending mode to lighten.
I, too, would like an app with exposure time longer than 4 seconds, but someone just told me that it might cause damage to the sensor or something. Maybe it could be done in the homebrew community; I know at that point it's in "at your own risk" territory, but for the sake of getting some really amazing night shots I think it might be worth the risk, maybe 10 seconds max or something.
As for the "just stack the images together in Photoshop" option, that doesn't work with every low-light photography scenario.