According to the Daily Telegraph[ The mouthpiece of the uk Conservative party... in an IQ TEST Harry only scored four whilst william scored seven , that does indicate he is not the brightest.

I had to check that one out. It appears to come from here: Telegraph 6 Nov 2005 and does not give a statement that "Harry only scored four". His score appears to be unknown, and the article relies on hearsay.

Prince William is the brains of the family - it's official. The 23-year-old prince, who is to enrol at the Sandhurst Royal Military Academy in January, scored seven out of 10 in the "raw intelligence" test, when he took part in a gruelling assessment at the Regular Commissions Board in Westbury, Wiltshire.

Prince Harry is understood to have scored four in the same test in September last year.

The article also gives some background of the test: A-levels get very high scores. A score of 10 out of 10 means you are a genius, while one or zero means you are barely functioning. Seven, which William received, is very, very good. He also displayed good leadership skills. On the showing he gave at the RCB there is no reason why he shouldn't make a very good officer."
This seems to show that William is exceptional, but does not give any guidance on what a score of 4 would represent, even if that were Harry's score.