If you plan to expand your small cctv system in future I would recommend to use a cheap network IP cameras, like AXIS 206, or Panasonic BL-C1 (this is in case of indoor usage). Also you can purchase and install some not expensive software. Very cheap one is
ActiveWebcam but not all people happy with it. If you ready to spend more try
Luxriot.
Mentioned cheap IP cameras have not replaceable lens with determined viewing angle. In some cases like you you need cameras with other angle (focal length) or vario-lens.
So calculate required lens "focal length" using lens tables or
lens calculators.
Also you need to calculate video archive size. 4 cameras 640x480, 6 FPS give about 70 gigs of video per day in case of constant recording.