I would then ask you how many days of drying time per coat. You said 4 coats, that means one day or more drying time depending on humidity per coat. Did you do that????
Poly takes forever to dry and will dry faster when the outside conditions are more favorable.
I think you have a problem of the below 3 coats were not allowed to dry thoroughly and they are now marshmellowing on you. You will never get a dry because the top or fourth coat has dried and sealed the other three and you may have a big problem.
How long did each coat dry and what was the outside conditions??????