6 point minimum IMO - 4 point is either a rear cage or a roll bar - the (ever ongoing / on hold Mini project for my daughter) we put a 6 point FIA approved cage with the added diagonals and door bars.
But it depends what you want it for - to make the car stiffer for handling and to give a mount for harnesses a 4 point will do that - to keep you shaken but not stirred in a major off then go 6 at least
EDIT: PS Watching with intrest - great project
Usage will be mostly road, spirited driving, some track use (but not really flatout type of driving), etc.
I dont want the cage interfering with the front of the car for getting in/out and just because I will see it. I also think a front cage will detract in the event of a future sale.
Harness, rigidity and some additional impact/rollover protection is good enough (I hope!).
Then rear half / 4 point should do all you need
I just like having an FIA approved cage round me when I'm on track - mainly cause I have fast cars but I'm not a very good driver