If the chip is CMOS, rather than CCD, then long exposures will be noisy. In astrophotography, people tend to use CCD chips because of this. CMOS chips are improving. I'm not sure what the n900 has, but I expect it is CMOS, as these are cheaper (CMOS are what you get in webcams). The chip seems quite small as well. I have used webcams and DSLR's. Webcams have small chips, and can produce reasonable results for planets - although CCD is better (Phillips used to make one, but they are hard to get hold of now). DSLR's are OK for Deep Sky Objects - although I have managed to use a Russian software hack to get reasonable images of Jupiter with one. the DSLR has a larger sensor. My guess is that you would get results with planets, but stars would be difficult. Light sensitivity is another problem - and with longer exposures you would need to start tracking accurately, otherwise you will get star-trails. I forget now how long my exposures were for something like Pleiades or Orion, but it was an hour of 30 second exposures to start getting any nebulosity out. 4 seconds without tracking, I'd have thought you'd be lucky to get a blur.

Edit: Just checked - yeah, it is CMOS