Halfway between 3 and 4 is what crews row at, as it is equal to the water resistance, as bach said. Especially starting out, lower is better. You can still pull the same times, but it reduces the potential for damaging your knees/back until you can master form.
This thread is the first I've heard of 3/4 being water equal... When I rowed it was 4+/5... Doesn't the Concept II site say 5?