Several factors go into deciding which configuration is best for your needs.

These 3 are the first key factors to consider.

1st - Recording Speed - FPS (Frames Per Second) While each camera has the capability of recording up to 30FPS, (considered "Real Time"), the computer card's capacity will limit each camera's capacity. Card capacities are 30, 60, 120, 240 & 480 FPS. If you connect 4 cameras to a 30FPS card, each camera will max out at 7.5FPS, resulting in a very jerky motion. 4 cameras would require a 120FPS card in order to get the full potential out of each camera ("Real Time").
2nd - Resolution of record and playback
While many systems may have a recording resolution of 640x480 lines, they may only have
a playback of 320x240 lines, making playback not as clear as you might expect.
3rd - Total capacity of the hard drive.
Each camera recording at 30FPS requires about 1.5 GIG of hard drive space per 24 hours.
For a full 30 days, 4 cameras at full capacity would require a hard drive with a minimum of
180 GIG available space for data storage, not including the operating system or software.

Before you buy - be sure you will get what you expect!