In most situations, a tower shield provides the indicated shield bonus to your Armor Class. As a standard action, however, you can use a tower shield to grant you total cover until the beginning of your next turn. When using a tower shield in this way, you must choose one edge of your space. That edge is treated as a solid wall for attacks targeting you only. You gain total cover for attacks that pass through this edge and no cover for attacks that do not pass through this edge (see Combat). The shield does not, however, provide cover against targeted spells; a spellcaster can cast a spell on you by targeting the shield you are holding. You cannot bash with a tower shield, nor can you use your shield hand for anything else.
When employing a tower shield in combat, you take a –2 penalty on attack rolls because of the shield's encumbrance.
Tower Shield Proficiency
You are trained in how to properly use a tower shield.Prerequisite: Shield Proficiency.
Benefit: When you use a tower shield, the shield's armor check penalty only applies to Strength and Dexterity-based skills.
Normal: A character using a shield with which he is not proficient takes the shield's armor check penalty on attack rolls and on all skill checks that involve moving, including Ride.
By RAW it looks like you use the 'total cover' protection, even if you don't have proficiency with the tower shield.
Characters who do not have proficiency can use a tower shield in combat and get the +4AC, but they take an extra -10 to all attack rolls from the ACP in addition to the -2 everyone gets.