I have a Sunpak PZ5000AF.
From the looks of it, this is the model down from mine.
While I'm not an expert with the 4000, here's what I do kow about the 5000:
It will not give you E-TTL. Only A-TTL, which is only usable with older film bodies.
It has an auto aperture mode (similar to using a Vivitar 283/285).
It was a really good manual mode with 1/3 stop adjustments.
It has an optical slave mode that will fire with other 1-burst flashes, like other manual flashes or studio lights, but it won't sync with a pre-flash type of unit.
Again, that's my 5000, so I'm assuming the 4000 does most of that.
In a nutshell, don't buy it thinking you can shoot TTL on a digital body.
This particular model appears like it will only give you full manual or 1/16 manual, as the TTL won't work and I don't see an auto mode option.