Hi Bobby,
Well, there are many variables that can affect the amount of brilliance a stone within a setting exhibits, but a great deal of the equation is going to be determined by the quality of the cut. The better the stone is cut, the more light will be returned no matter how the stone is set.
The vast majority of light that a well-cut diamond returns to the viewer is light that enters the diamond through the table (top) from above, is reflected off the pavilion facets, and is returned to the viewer back through the table and crown. A good illustration of this phenomenon can be seen by viewing the fantastic brilliance of a well-cut diamond in an enclosed "bezel" setting that completely surrounds the girdle and blocks all light from entering through the sides, which I believe is what Lugus was referring to. In an open-prong setting there will be a negligible amount of light that will enter the stone through the pavilion and leak through the table to be seen by the viewer, but this light should be of little consequence as it will typically be overwhelmed by the light being reflected off the pavilion facets (how often is the light source below the ring, anyway?).
Having said that, as the angles get a little less "ideal" and the proportions stray, a diamond will be more likely to be affected by ancillary light entering through the pavilion from the bottom of the stone, but under most circumstances the light you're seeing is reflected light. The difference between a four-prong and six-prong setting might affect the overall luminance of the stone, but that would usually be more indicative of a less than well-cut stone than anything.
Lugus brings up a couple of good points in that you may be witnessing a reduction in brilliance due to more of the crown of the stone being covered by the tips of the prongs. Also, not to be alarmist, but he also mentions a good point about the possibility of the diamond having been switched. Chances are you still have the same diamond you always did, but if the difference you're seeing is pronounced you may want to try to confirm that the diamond wasn't switched while being reset. It's not common, but sometimes unscrupulous jewelers will take advantage of customers this way. Maybe it's better to be safe than sorry.
-Tim
