If you install harnesses, you should keep your OEM 3-points installed as they're much better to use on the street. Plus most 4-points aren't DOT approved and will make checking your blindspots quite difficult. Personally, I'd stay away from 4 points since the lap belts tend to ride up on you and don't provide the same protection in the event
Ideally, you really need a roll bar to do harnesses right, but a lot of people choose to use only a harness bar. Keep in mind that many instructors will refuse to ride in your car if you have harnesses without a roll bar. Also, the golden rule is that you afford the instructor (or any passenger for that matter) with the equivalent safety equipment.
I'm currently using a set of Schroth 6pt Profi-III ASM harnesses, which I only use on the track. On the street, I always use my seat belt, even though my roll hoop makes the reach a little cumbersome.