I tested this on netherite armor with three pieces unenchanted and one that had fire protection by jumping in lava and timing how long it took for me to get to half health. I found that it took me a lot less time to get to half health with fire protection 4 than it did with levels 1 2 or 3. It seemed to take about half as long with level 4 than it did with level 1. I tested it with the enchantment on one piece at a time and with different pieces and even with diamond armor and got the same results. It also did this when standing in fire instead of lava.