If a player signs a five year deal, when that contract expires he is not a restricted free agent and the team must either franchise him or let him go into unrestricted free agency.
Lower picks in the draft usually sign for 3 years, and then the signing team gets the option of extending them for another year at a restricted free agent price. Basically rookies are exclusive rights free agents even after their contracts expire if their rookie contract is signed for 3 years.
All it takes is $1.4 million and it's like a 3rd year player mini-franchise tag for restricted players. Like the Patriot's have done with David Givens this season and what the Bengal's did last year with Rudi Johnson, tender them the $1.4 million offer so that if another team wants to sign that player they have to give up a first round pick to do so.
Restricted free agents offers are 1/5th the cost of a franchise tag and almost as powerful (give up one 1st rounder instead of two 1st rounders for a franchised player).
5 year contracts give you added value to the team compared to a 4 year contract because the signing bonus gets extended for another year, and more importantly the player wouldn't be able to hit the free agent market after year 4. Top picks lose free agency after year 4 in exchange for more upfront signing bonus and guaranteed money and high base salaries. The more money (especially guaranteed) that they receive, the longer their contract will be...
Walker is going into his 4th year in 2005. If he had signed a 4 year contract as a rookie, he would have been an unrestricted free agent after this year. He could have suited up in week10 to gain credit for this year and then been free to test the market. But since he was a first round pick and signed a 5 year deal, he had to settle for the $2 or $3 million dollar signing bonus he got upfront and wait until after year 5 before he could hit the market.
Sometimes highly touted rookies get signed for 6 years or more if the team gives them enough incentive to put off the inevitable free agent payday (Alex Smith $24 million in guarantees). I believe Sean Taylor last year got duped into signing a 7 year contract for chump change, so now by the time he's a free agent his big pay day will likely have come and gone the way his legal troubles have been shaping up.