1) No rollbar, no harness. In the event of a possible rollover, you will held sitting up with a roof smashing in on your head. With a factory seatbelt, your body have some degree of movement which can be safer.

2) A 4-point harness is what we like to call a "tuner" harness because they're not good for daily driving or for driving events. They are worthless aside from car shows.

Certified race harnesses have that fifth sub-strap there for reason. When buckling the harness, the sub-strap is there to prevent the lapbelts from rising up over your stomach. The lapbelts are designed to sit over your lap and hips, not your stomach. Should the harness rise up over your stomach due to a lack of a sub-strap and you get into an accident, the harness will be restraining you against your vital organs. This can lead to a serious injury.

3) Measuring, bolting, and fitting a complete 5-point harness should take you no longer than an hour depending on if you need to drill new mounting holes or not. If so, also look into purchasing a backing plate for the mounting point.

I can sell you a G-Force Racing Gear SFI/FIA certified harness for a good price if you really want, but I do not condone using these on the street.