4GB on Vista might be straining for an image that size. If you were in XP, you'd have an easier time of it. Windows 2000 would be better still. Seems backward, doesn't it? If you start to have problems, try turning off Aero and any other unnecessary bells and whistles. Vista seems to have pretty good memory management in my experience, but it's still rather bloated.

Reducing the color depth, though, will help you a lot. Are you really talking black-and-white (1 bit) or grayscale (8-bit)? A 1-bit image is going to be light enough that you'll probably be fine even at the huge size you're talking about. An 8-bit grayscale is still better than full color, as it has only 1/2 the amount of information that an RGB image has (assuming that you're working non-destructively. 1/3 if you're not using masks.) Just make sure you're actually in greyscale mode, though, or you won't get any of the benefits. Image > Mode > Grayscale.