Key #3: Kenneth Abraham
The Maroon Assassin
When you reach Kingston, you can find your next Assassin pal Anto at the spot marked above. He'll first task you with eavesdropping on a conversation. Keep a safe distance while the men speak, then move across Kingston to another set of guards that need to be eavesdropped. After the second conversation, the guards will sprint away. Quickly tackle one to end the mission.
Return to Anto for a second mission - this time he wants you to help free some slaves. Start by hiring the drunkards to battle the guards dead ahead (alternatively, you can rush in and just kill them all), then free the slave inside the restricted area. Repeat this process for the other two slaves: one is inside a fenced-in restricted area, and the other is being escorted by guards. Free all three to clear the mission.
Ready your weapons and meet with Anto again. This time, you have to defend the bureau from waves of guards. You'll have chances between waves to thin out the number of guards in the next attack. Use your combat moves wisely and quickly, and you'll clear the mission's three waves without any trouble.
The Commander's Ruse
Return to Anto for his final mission, where you'll have to track down the man responsible for the slave attacks. Fun fact: he's also the guy with the next key. You'll start by tailing an increasing number of guards. When the group splits, keep tailing the duo in blue. Once they lead you to the plantation, you have to locate Abraham. Use Eagle Vision to locate him in the northeast, by some farm buildings. Kill him off to obtain his key.
Key #4: Lucia Marquez
The Taino Assassin
The final key is obtain during missions from Opia Apito. You'll find her on Cayman Sound, located at 327, 334 - the island is heavily guarded, so be careful or battle-ready when sailing there. Her exact spot on the island is indicated above. She will challenge you to a hunting contest. To win, simply skin at least one more animal than Opia does - the trick is getting it down quickly. Once you do so, kill the white jaguar that shows up to complete the mission.
Opia has now moved to Grand Cayman, located at 397, 324. Fortunately, this port isn't guarded, so sailing there is easy. Speak to her at the docks to begin the mission, then follow her through the port. You'll then have to use Eagle Vision to locate a Templar called Alvin. Do so, then eavesdrop on his conversation. When it ends, pickpocket the indicated man to complete the mission.
Opia is still in Grand Cayman, so return to her for the next mission. Follow her and duck into the nearby leaf pile when she says to hide. When your target runs, chase him until he escapes to his ship. Board the Jackdaw and take down the ship, then board and take him out. Now you can go after the true key holder.
The Trail of Lucia Marquez
You'll find Opia one final time on Pinos Isle, located at 345, 469. The isle is guarded by some ships, so be careful in your approach. Meet with her on the beach to start the last mission. You'll first tail a soldier into the ruins. Here, you'll have to take out some guards - dispatch them quickly by shooting the nearby powder kegs. Battle the second wave of guards that appears, then chase down Marquez. Take her down to snatch the last Templar Key.
The Templar Armor
Now return to your base on Great Iguana. Head up to the manor atop the cliff James Kidd took you to during the mission This Old Cove. You can now open the five locks and obtain the Templar Armor inside. Enjoy your 25% damage reduction!