In my testing, the different protection enchantments don't stack effects on armour. The most noticeable was using Protection 4 and Blast Protection 4. With just Protection 4 you take 4 hearts of damage (8 damage), with just Blast Protection 4 you take 2 hearts of damage (4 damage), but with both Protection 4 and Blast Protection 4 you still take 2 hearts of damage (4 damage).

At first, I thought it might be because I hit the armour's maximum protection percentage, so I added more TNT (to make the setup in the video below), and the numbers were still the same.

https://youtu.be/Qi4vUn9kMQg