Potassium phosphate buffer + EDTA

Given 10 mM potassium phosphate buffer (pH = 7.0), 50 mM potassium propionate and 20 mM EDTA (acid). After addition of all 3 components pH becomes 4 since EDTA has 4 $\ce{OH}$ groups.


How to calculate the ionic strength if I am pHing it with $\ce{KOH}$ up to 7.1?
I can calculate ionic strength for binary mixtures, but 4 different components makes me stuck.


