As far as I'm concerned, Roger Staubach, Bob Lilly, and Emmitt Smith are locks. Michael Irvin is in a group of 3 to 5 that have a valid argument for being that 4th guy. I've already mentioned Bob Hayes.
Mel Renfro was a Pro Bowler for 10 consecutive years beginning as a rookie, he was a shutdown corner before the term was ever phrased. He was extremely dangerous in both kickoff and punt returns, he played for 14 years, played in the two NFL championship games against the Packers and played in 4 Super Bowls. One year they called upon him to carry the ball as a RB for two games and he averaged 6.5 yards a carry.
OLB Chuck Howley was Sean Lee without the injuries, a consummate playmaker. The bigger the game, the more he contributed. The original #54 was a Pro Bowler 7 consecutive years and an All-Pro 5 consecutive years. He is known mostly as the first Super Bowl MVP that played defense as well the the first and only to play for a losing team. However, it has been argued that he should have also been the Super Bowl MVP the following year. In that game, against the Dolphins, Howley recovered the only fumble Larry Czonka had the entire season. In the 4th quarter, with the Dolphins at midfield, he intercepted a pass, running it down to the Cowboy 9 yard line.
Only Bob Lilly made more Pro Bowls than Larry Allen, only two Cowboy players were selected more often as All Pros. Larry Allen made the Pro Bowl 10 of his 12 years as a Cowboy. But this is only half the story. Larry switched positions 4 different times, in 1994 he started at RT, the following 3 years he started at RG, then LT in 1998 before finishing the last 7 seasons at LG. He made 1st team All-Pro at 3 different positions. Unfortunately, his dominance took place mostly during the Cowboys struggling years, he started only 1 Super Bowl. Otherwise, I think he would have been the unchallenged 4th Cowboy hands down.