I've named mine (ZF4) "The Brick". I love that it's a thinner/narrower width, but that does come with increased depth/thickness. Nevertheless it's a really interesting phone, fast, great great visuals on the front (outer) screen, and it's fun to when needed flip that thing open and get this great expanse of screen.
Not to be a downer, but I'm not sure how much $$ you'd get on a trade in , but I think the ZF4 is a fun phone with many upsides. And, if nothing else, it makes a great self-defense tool if you're out at night and someone says "Gimme all your dough!". WHAP! Instant ZFold4 Justice, baby!