In order to claim a player on waivers, a team with a lower priority; in your case positions 1, 2 & 3 would need to also add the player. The system does not ask the teams ahead of you on the waiver list if they want to claim the player, instead you both put in the claim and when the player passes through waivers the claim goes to the team with the lowest waiver priority. Now if you were to get the player, you would no longer be 4th in the waiver position, you would move to the last position.
As far as adding players, you can add whomever you want, you could drop a QB for a DEF or a K for a RB, there are no restrictions other than if the player is on waivers then you need to wait out the waiver process. If the player is a free agent your free to pick him up immediately and the player you dropped will go to waivers.
Hope this helps, good luck.