A CE-560XL flight crew suffered an NMAC with an unknown VFR aircraft while descending as cleared by TRACON.

Narrative

I was the pilot not flying during descent and initial approach into CCR. We were talking to Travis TRACON. We were at 6;000 MSL and were given a descent to 4;000 MSL. The pilot flying initiated a 1;400 FPM descent. Approximately 20-30 seconds later Travis notified us of traffic at our 12:00 and told us that we needed to descend 'like he told us to.' We replied that we were already through 5;300 FT and descending at 1;400 FPM. We then immediately got a TCAS alert for traffic that was at our altitude within 1 mile of our current position followed by a DESCEND RA with which we complied. Since we were in a descent to begin with; the traffic was initially under us and we could not see him. We descended and recovered at about 4;000 FT MSL. Very excessive force was required to prevent the collision. I would estimate the distance between the two aircraft at no more than 200 FT lateral. I asked the Controller after landing if the traffic was IFR or VFR. It was reported as being VFR. Now; I know that the Controller has only a responsibility for IFR traffic separation; however; he descended us directly into another aircraft with no adequate warning. It was dusk and difficult to see anything. In my opinion; this Controller was responsible for this event and responsibility or not he has an obligation to not descend us through an altitude that put us on a known collision with another aircraft and then give us no warning prior to the TA.Since I had no visual on the other aircraft until after the incident; there was no way I could have seen and avoided this conflict. The Controller needs to be more responsible with traffic separation.