You have stumbled across a mysterious letter filled with gibberish.
At the top of the letter, you see this clue:

"Each letter corresponds to only one hidden letter with no specific rules on how the hidden letters are assigned. The hidden letter of B is I."

At the back of the letter, you find 4 encrypted English words:

Using these clues, your goal is to determine all the hidden letter and then decrypt the following word:

Good luck!

Note: You are allowed to use a dictonary or search the internet to find words or information, but the final words are not difficult so it is not required.
Note 2: The intended way of solving this puzzle is through observation and logic. You can definitely use a substitution cipher solver, but that would ruin the fun of the puzzle :(