I had a DP4 a while back. I liked it. Not sure if it was my rig, or the unit, but there was always some kind of weird digital distortion going on in the background. Impossible to hear when playing with a band, but when noodling alone, kind of got on my nerves a little. Very flexible unit, but it's definitely old school now. I've actually got a DP/2 right now that I need my tech to take a look at to help solve a hum that just appeared one day.
I'd say for a DP4 (not+) that I wouldn't spend anywhere over $200. Sure they're "rare" but that doesn't necessarily make them valuable.