MS Excel: How to get a string of repeating letters from a bigger string?

Given the constraints on your data, where there is only a single transition, you can just repeat the rightmost character the number of times it exists in your string:

=REPT(RIGHT(A1),LEN(A1)-LEN(SUBSTITUTE(A1,RIGHT(A1),"")))

Not sure if the "four letters" refers to a data constraint or a formula function, but if there might be more than four letters, and you only want to return the last four, then embed the above formula in a RIGHT function: =right(the_formula,4)*

If the string can only change once, then all you need to do is "throw away" the left hand characters, then limit the remainder to 4. "Substitute" will swap a string for another, so you can swap the leftmost character for nothing, however many times it appears.

=LEFT(SUBSTITUTE(A1,LEFT(A1,1),""),4)

If the string could change zero times eg ooooo or xxxxxx then this will give an empty result. This might be considered correct if you only ever want the second letter since there isn't one.