At the risk of feeding the troll at unskewedpolls.com and barackofraudo.com (both run by Dean Chambers) or legitimizing its claims of voter fraud which journalists are thoroughly debunking, let me add one more reason why the claim that voter fraud flipped the results of Pennsylvania, Ohio, Virginia and Florida is absurd.
If the claims of fraud are right, Obama would have won Nevada, Colorado, Michigan, Wisconsin and New Hampshire but lost Pennsylvania, Ohio and Virginia. You don't have to be Nate Silver to know that the odds that Obama would win close swing states like Colorado, Nevada and Virginia, and at the same time lose a reliable Democratic state like Pennsylvania and other close swing states like Ohio and Virginia are very low. That's just not a plausible electoral result. If Obama is doing well enough to win swing states like New Hampshire and Nevada it's just absurd to think he would really lose a state like Pennsylvania. Chambers' "map" belongs in the realm of unicorns and birtherism.
Moreover, for Chambers to be right, the polls, which were reasonably accurate nationwide, would have to be (for some unexplained reason) wrong for only those four states. The whole thing is just nuts.
If you want to get nerdy, this comes down to Bayes theorem, which in very simple terms says that when judging the likelihood that something is true, we have to consider the evidence of its truth or falsity in the context of the baseline probability that that thing is true. If you say the sky is purple, I should be very hesitant to agree based on your anecdotal evidence because I already know as an initial matter that the odds that the sky is purple are extremely extremely low.
As applied the voter fraud claims, before we even consider Chambers' anecdotal (and highly suspect) evidence of voter fraud, consider it in the context that the map he is proposing is highly highly implausible. As a result, there is very little evidence that should convince anyone (thinking clearly) that voter fraud really caused the outcome he claims since that outcome to so unlikely. Yet, hope springs eternal.