Note: This trick requires the basic sniper rifle fully upgraded with the Exclusive Upgrade. Make a note of the dodge actions at various times during this battle. Start off by running to the side of the platform on the top. Quickly shoot the tentacle in front of you, making sure that you dodge if needed. With the upgraded sniper rifle, it will only take one shot for the tentacle to retract. Ignore Salazar's eye and other tentacles. Only focus on the tentacle closest to you. After some point, Salazar himself will emerge from the upper back of the monster. Use your sniper to pick him off. After you get about two shots in, he will go back into hiding. By now that tentacle should have returned. Shoot it once to take care of it for awhile. Repeat the process of shooting Salazar and the tentacle, and the battle will cause Leon to take little damage. If you run out of ammunition for the sniper rifle, either drop down and look for more in the barrels (but be prepared to fight the spider-like parasites or run pass them), or use your Punisher to snipe Salazar and just dodge the tentacle.
When you are in the castle, you may find a rocket launcher in the room where you must make a bridge to get to the other side. Take the rocket launcher but do not use it. Wait for your fight with Salazar to use it to end Chapter 4 easily. Immediately before the Boss battle with Salazar, take the TMP, the rocket launcher, some herbs, and a shotgun (Striker recommended, but the Riot Gun works). When you enter the room, watch Salazar fuse with the monster, then get to work. Use the shotgun or TMP on the tentacle to your left until it goes away. Grab the item that is at the end, then fire with the TMP until Salazar appears. Then, equip that rocket launcher and fire for a one hit kill. Get the 30,000 or 50,000 Peseta that he drops. This will pay for the rocket you used, and you will save health for the next chapter. Once the Pesata is collected, go down into the pit and pick up the remaining items. You can then leave and proceed to Chapter 5.