PF uses more classic D&D-isms than 4e (like spell names), but things like skill DCs scale in both PF and 4e, while being mostly flat in 5e.
Most of the named enemies in ZEITGEIST were unique, so conversion would not be a simple case of "Oh, I'll just use the 5e orc instead of the 4e orc." It's more like, "Huh. How do I model the party fighting a 15th-level fighter who's already taken out a hundred people and is down to only a few hit points?" In 4e and PF his AC was very high so the party probably needed a nat 20 to even hurt him, while each of his attacks probably dropped a PC. (The intention was to make the party try negotiation, to highlight very early on that the world has very dangerous people, and to teach them that not every challenge in this campaign is overcome by combat.)
I think the 4e design has more flavor, especially early on, when we were more tentative in our conversions to PF. Each 4e enemy had a brief description of all their attacks, and so I started putting in more interesting ideas. Rather than "he has a sword" it was "she says a command word and teleports a pair of pistols with bayonets into her hands, then snaps bullets in your direction while she dashes to cover" or "the druid gestures and one of the low-hanging fog banks drifts across you, ruining your vision, but you hear buzzing of wings from some unseen creature."