100 iso, long exposure. One way to find out.

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

This is one I prepared earlier - taken with a Canon EOS1000D, using a 4" ED refractor on an EQ mount

This was with a Sony Alpha, I think, and the ED refractor. The moon is the best place to start, because it is big and bright - but it does move faster relative to the stars and planets. Needs shorter exposures, less sensitivity.

This is the best I managed for Jupiter using the EOS 1000D with the video hack. Took a lot of post-processing to bring out any detail. I never thought of trying the n900 - as I attach the camera direct to the telescope and just use the sensor (avoiding the camera lenses). You can use anything with a sensor - I rigged up a CCTV camera to take these images of Mars & Saturn.

There might be another way of approaching this - with the CCTV camera, I hooked that up to a HDD recorder, then processed the video. You might find taking video with the n900 produces better results - you take the frames recorded and put them into something like Registax to stack them and average them to produce a better image. You'd still need to track the object quite accurately though.