Can I add oz of volume 20 developer with 4oz of 1 0 developer to make a total of 5oz?

No, you cannot combine 2 oz of volume 20 developer with 4 oz of 1:0 developer to make a total of 5 oz, as that would equal 6 oz. If you want a total of 5 oz, you could mix 1 oz of volume 20 developer with 4 oz of 1:0 developer. Adjust the amounts accordingly to ensure the total adds up to 5 oz.