If your cabinet is full size, you might want to consider 3" casters, as they will be much easier to move around under load than 2" ones, even though the 2" ones might technically be able to handle the weight.
Also, go with the 2 fixed/2 swivel method -- it will make getting your heavy cabinet into a stable position much easier.
I agree with the recommendation to avoid particle board -- it will never hold up under the lateral stresses of the bolts, and eventually the casters will be ripped out, even if you through-bolt them to a backer block. For the base, I'd use at least 3/4" plywood with a backer block of hardwood, as suggested above.
Personally, I'm going to mount these casters...
http://www.woodcraft.com/family.aspx?familyid=1989... on a base constructed as a torsion box. The casters will be through-bolted, and the spots where the bolts pass through the box will be solid wood.
-- Chris
P.S. Brief summary of torsion boxes:
http://www.workbenchmagazine.com/main/276-torsion.html