I'm digging into the Minecraft enchantment system, and looking at the wiki, I see the same type of protection can't be on the same armor type – but what about different armors? More importantly to me, can I wear several of the same kind of protection (fire, projectile, blasting) on different pieces and stack the protection effects? Does it follow that wearing 4 pieces of high valued fire protection will make me almost immune to fire damage? Or will I just get the effects of the best one with the rest going to waste?
Yes, fire protection stacks. To test this, I put on a diamond helmet with Fire Protection I and stood in fire. Then, I put on an entire suit of diamond armor with Fire Protection I and went to stand in the fire again. I took no damage, though my armor took extensive damage.
This is the video I made in the process (I'm not sure I've properly annotated it, but hopefully you can tell what I'm doing. :P):
They definitely stack, I just tested them. You're pretty much immune to fire with two diamond pieces of fire IV if your hunger bar is full.