I think in this case you also need to keep in consideration the team psyche and morale issues. The players understand it's a cutthroat business but you have to show you are rewarding the behaviors you want. By all accounts Travis has worked very hard to improve and seems to be popular with teammates and a help to the other qbs. He's finally getting to the point his play may improve because of his years in the league vs a 34 year old qb.
I think you have to keep 4 for that reason. It's a cutthroat business and at other positions it may not be as much of a consideration but if you go over the line regarding how you treat some players I think you lose some of the ability to keep them focused as a team.
For ex., most people think Parcells is cutthroat but look at the players he brings to his teams. He rewards players who give him what he wants so clearly that most players get on board. Parcells is loyal to you if you are loyal to him and all the great coaches are like that. Crusty old Lombardi was the same. With Mularkey's experience as a player I think (I hope) he will build a team in the same manner. I am willing to bet right now Brown will not get cut. Zolman is gone and we'll carry 4 qbs. You can mock my comment if I am wrong!