I am having a little trouble understanding how the protection enchantments work, so my question is, will it help any if I put Featherfalling IV on my Protection IV boots, or does that exceed the cap? All my armor is diamond and with only protection IV
The reduction from enchantments is based on the Enchantment Protection Factor (EPF) of any relevant enchantments. The EPF of an enchantment is based on the level of the enchantment and the type- protection enchantments have a lower epf than more specific protections such as feather fall; they make up for it by working against anything.
The EPF of Prot IV is 5 and the EPF of Feather IV is 18. The cap is 25- this means a full set of Prot IV doesn't quite hit the cap.
If a single piece of equipment has both Protection and a specific protection, such as feather fall, only the best applies. This means a single Prot IV, Feather IV item will count as 18 against falling- not 23.
Adding Feather Fall IV (or Feather Fall III) to one piece would be enough to take you over the cap for falling damage.