Ooh, that's a tough order. You could check this Extension publication on shrubs and search the text for "hedge" and/or "evergreen". Good luck!

Would it be possible to make a hedge out of Degroot's Spire Arborvitae?

Degroot's Spire was my thought until I realized you said 4 ft high. Not possible. Are you sure you can't just let the go? Keeping them narrow would be easy. Prune them to one leader, or multiple that are parallel with the fence.

70' long is kinda long--lots of plants would be needed. Maybe your neighbor would be receptive to planting a vine on the fence.

Here's some possibilities...

Here is a link that might be useful: Varieties of Arborvitaes

Hi Leftwood and Leaveswave,

I wouldn't need to keep them at exactly 4 feet high-they would just need to grow at least that high. How well do you think they would handle part shade? Also, does any one know of a cheap source for Degroot's Spire that isn't 6 inches high? The best deal I've found so far is an 18" shrub at fantasticplants.com for $14.95 + shipping.