Adam, you will want at least 500 ml of PO solution to develop your 4x5, personally I would use at least 1 liter to 1 1/2 liter....you reuse PO so really might as well insure you get the print covered completely.
The problem is that you will have to make at least 5 different solutions of PO/Pot Di to have enough contrast spread. I use solutions of PO at 30% so if you plan to do the same you will have to order 750 grams to make 5 different 1 liter solutions.
I use a 10% solution of pot di to be added to the different "contrast" solutions of PO. The way I do it is 1 ml pot Di/200 ml PO for solution 1
etc, etc....so for one liter this would be the amounts
Developer 1 1ml PD/200 PO= 5ml of PD/1 liter of PO
Developer 2 2ml " " = 10ml
Developer 3 3ml = 15ml
And so on...I am sure you get the idea.....
You will have to determine a replenishment rate of the pot dichromate, IOW, after you use the developer certain number of times you will have to add more pot dichromate per 200 ml or you will start seeing a lowering of contrast.
Yes, you use only ferric oxalate number 1. For example if your initial drop count was 6 drops of pt/pd, 3 drops of # 1 and 3 drops of #2 you would use instead 6 drops pt/pd and 6 drops of #1.
I disagree with cjarvis, there is no such thing as "too" much sensitometry and testing when we are talking about pt/pd. If you follow Arentz, you will have a methodical approach to printing that will allow you to "guesstimate" printing times and contrast right from the first print which will result in significant savings.