To get the final bottle, you have to go Poe hunting.
There are ten Big Poes hidden around Hyrule Field. You will need an empty bottle to catch them. It helps if you're riding Epona to find them, so play Epona's Song and climb into the saddle. At each "hot spot" listed below, you should just stand still on Epona when the Poe appears. It will fly backwards away from you, then disappear. If you didn't kill it, it will simply reappear and fly backwards again if you stay still. The best strategy to kill them is to turn in a direction such that the Poe flies in basically a straight line away from you. Shoot the Poe with two arrows to kill it. Once you kill it, ride up to it, hop off, and walk up to it, and you'll be asked if you want to catch it in a bottle (say yes!). Then go to the former guardhouse in Hyrule Castle Town and sell it to the creepy guy (show him the bottle) for 50 rupees and 100 points. Then go back out to Hyrule Field and continue the hunt. After giving the guy the tenth poe, he'll give you the fourth and final bottle!
Big Poe Hot Spots
- The ledge above the river between Kakariko and Zora's River. You can stand underneath the ledge if it puts you in a better position to kill the Big Poe.
- The crossroads in front of the Hyrule Castle Town entrance.
- The angled part of the wall east of Lon Lon Ranch.
- By the stone on the path south of Lon Lon Ranch, just before the fence.
- At the crossroads near the Gerudo Valley entrance.
- The bush nearest the tree by the Gerudo Valley entrance.
- Stand in the circle of bushes near the small copse in the southern area of Hyrule Field.
- By one of the trees in the small copse in the southern area of Hyrule Field.
- By one of the bushes west of Hyrule Castle Town, near where the river is flowing.
- By the tree at the Lon Lon Ranch entrance.