I say two of each male and female.
When a hatch is staggered with more than 24 hours between chicks hatching the hen will often get up to take care of the new peeps and the other eggs get cold and die. It is better in the wild to mind the ones that have made it then to wait for ones that may never hatch. At 24 hours I take the rest of the eggs candle and set them myself.
What you would probably find on breakout is developed chicks maybe even piped into the air cell that got cold, especially if you heard one peeping. Since I always find that depressing I only do it on hatches gone bad. If I get a hatch on shipped egg less than 50% or on homegrown eggs less than 80%. Then I feel I owe it to myself to find out what I can about what may have gone wrong. If all of the peeps reached a certain point of developement... say day 15-16 then died I can be pretty sure that there was an incubator/broodie issue at that time... temp spike, power outtage, lid slipped off, grandkids played with the bator, broodie left the nest for too long... I have had them all.
I think that 4 chicks from 6 eggs is a good hatch. Congratulations!
Melissa