4MP Cameras are HD Quality.
4MP resolution are usually 2592 x 1520 effective pixels.
Cameras must match your DVR, for example a 4MP camera can be used on a 5MP DVR - but a 4MP camera can not be used on a 2MP DVR.
Cameras come with different lenses, most commonly;
2.8mm Lens (Wide viewing angle)
3.6mm Lens (Not as wide, but still a good coverage)
6mm Lens (Zoomed in)
Variable (VF lenses allow you to adjust the lens to suit your needs perfectly, example adjustable 2.7mm to 13mm)