4 feet tall is very low for this plant so you would need to train it to grow sideways. In which case 1 or 2 would fill the fence.

Thanks floral, I might add height for it to climb.

I hope that the planter is large and deep to accommodate the roots of these. They are great plants, and I find the hummingbirds really like this species, Lonicera sempervirens. I'd probably put in 2 or at most 3 and plan to both train sideways and add height to the fence in the form of stout 8' trellises.