Geof zone 4b is pushing it even with protection. Pot culture is basically your only option if you want healthy plants. A 15 gallon pot can house a fig tree for a while just remember to root prune your rootbound plants every few years and you'll be fine. Also remember to move your pots several times a year so they don't send roots into the ground making it hard to move in winter. Store your potted trees in an unheated shed, building, or garage during winter. Figs can be grown in ground out of there rated zone and will die to the ground but often come back to produce a main crop however you'll lose your breba crop if last years wood keeps dieing. For the record all fig trees even hardy chicago is rated for zone 8 however some figs take cold better than others Marsellis VS black is very cold hardy along with Hardy chicago and a few others but none are rated for zone 5, thats a sales gimmick. Established healthy trees can survive lower temps than young trees but I think zone 5b is as low as I'd go with in ground plants with winter protection anything lower and they will need to be brought inside during winter. Even in my zone 7b I have dieback on some of my fig trees but I never use winter protection because I have hundreds of fig trees and it's too much work. My first 2 years of collecting fig trees I would store potted trees in the greenhouse but the heat caused them to break dormancy early and then I was stuck using supplemental heat during the night to keep the new growth from dieing. Now I line all my potted plants against the house in winter and let them be and the ones in ground overwinter unprotected.
