The East island has three safehouses with garages where you can store one car each:
- Links view
- El Swanko Casa
- Ocean heights
Here is a simple method to obtain Four Everything proof(EP) Admirals and keep them.
1. Trigger & fail the mission three times and each time take the EP Admiral and store inside the safe house garages in the East island mentioned above.
2. Again trigger the mission for the fourth instance & go through all the stages except the final phase where you have to give Diaz the Briefcase you picked up which ends the mission successfully. Diaz will be sitting in his Admiral which will be facing the South and the pink marker which you have to enter will be at the left side of the car.
3. Approach & enter the pink circle from the north moving in a southward direction parallel to the car and quickly move forward just as the cut scene ends which gives you a mission completed message. Then take a left turn on the road intersection and you will (always if done correctly) find in the middle of the intersection an Admiral (which will be the same which Diaz drives).
I made a short video of this procedure.
This Admiral is extra special than the previous three you obtained because of its spike proof & bullet proof tyres until you put it in a garage. You can use pay'n spray safely since it does not take away those special attributes. You can also save your game by parking it some where near by your safe house making sure the car doesn't disappear.
4. When you step out of the vehicle subsequently you will receive a call from Diaz to meet him at his mansion in Starfish island which is unlocked now. Go ahead and meet Diaz who will give you the next mission "The Chase" where you have to pursue a gangster and this car makes it a lot more easier.
5. "The Chase" mission begins in Vice point at a house near the Jewelery shop. The culprit you have to chase, runs from his hideout over rooftops in a northerly direction and jumps off to escape in a BF injection parked near a Faggio scooter. Before entering the pink marker at the house which triggers the chase, park the Admiral near that very same Faggio, facing East, keeping the doors open. When you pursue the BF Injection, the gangster will be shooting at your car with an assault rifle & the great fun thing is, that your car will take all the punishment without incurring a scratch or a dent thus making this mission a piece of cake to pass.
6. After passing this mission go again to Diaz' mansion in the same Admiral, parking it near the bottom of the stairs and enter the pink marker which triggers the next mission "Phnom Penh 86" where you will be handling the machine guns of a helicopter. When you complete the mission you will be transported back to the front patio of the Diaz mansion with your Admiral sitting nicely at the same place where you parked it before the start of the mission. West island is open now henceforth.
7. All you have to do then is to take the Admiral to Downtown in West island, purchase the Hyman condo safe house and put your special car in one of the three garages. So now you will have Four everything proof Admirals at your disposal.
Edit:Refined this walk through a bit.