hi folks,

just curious, when (in what situation) do we set the shutter speed to 4 sec on a SLR camera?

When the SLR is on a tripod, and you set the timer or use a shutter trigger---and of course, when the light is low requiring a 1/4 sec shutter speed. I've had the shutter opened as long as 30 seconds for some night shots (gotta use a light meter though).

If you want to shoot light trails from car lights and such a long exposure is required to cause the light to streak. 4 to 15 seconds seems to work best. For star trails you need to have exposures as long as 5 hours.
Last night I was shooting Tugboats with their lights on and the exposure was 25 minutes at F8 on Fuji Velvia 50. This was my longest exposure I have tried on my Nikon F80.

Happy Shooting
Alex