I have seen this problem on Mitsubishi and many other brands of DLP TVs, as well. Most of the time the entire light engine is either replaced or worked on. Each spot (pixel) that you view in the picture is made by a single mirror moving back and forth on the DMD. In total, there are over a million of these tiny mirrors, and the white spots you see are those mirrors that have stuck in the "On" position. These mirrors get jammed and hence this problem.
I recommend contacting Mitsubishi Consumer Relations at(NNN) NNN-NNNN to ask (emphatically insist) that they cover the cost of the repair. They may very well supply the optical engine(part no. 938P075040) at no charge(as it is premature failure, c'mon the set is only 4 years old), with you paying the labor; but, it doesn't hurt to keep trying for the labor cost, too. Sometimes they will offer you a rebate on new set. If your set is under extended warranty then you won't have to spend a penny. Else I would guesstimate this repair would cost you around $1000.
They will have to come and check the set onsite first before they can offer a concession. Hence you will have to call them and place a request.
Hope it helps. If you have any other queries then I would be glad to assist you further.
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