Think about it this way.
The cap hit of Coleman's contract averages out to $4 million per year, but is probably much lower than that in the first 3 years.
In 2007 the cap is $109 million for 51 players. Let's assume that the non-starting players have an average cap hit of $1 million per player. That means that non-starters take up only $29 million of the $109 million cap. Thus, $80 million is available for the 22 starters on offense and defense. That means your starters can have an average cap hit of about $3.6 million per year.
When you think about it that way, if the Jets expect Coleman to be a starter, and they probably do, then Coleman's cap hit is about the average for a starter. Actually, it's probably much less than average in the initial years, and will continue to be less in the later years, because by year 3-5 of his contract, the cap will have increased significantly (thus increasing average pay for starters).