Check if last character in series is upper case and convert to lowercase if true

I have a column of strings that look similar to the following:

1          IX-1-a
2          IX-1-b
3          IX-1-C
4          IX-1-D

Some end in lowercase letters while others end in uppercase. I need to standardize all endings to lowercase without affecting the letters at the beginning of the string. Below is some code fragment that I am working with to make changes within the series but it doesn't quite work.

if i in tw4515['Unnamed: 0'].str[-1].str.isupper() == True:
      tw4515['Unnamed: 0'].str[-1].str.lower()

How can the truth table from tw4515['Unnamed: 0'].str[-1].str.isupper() be utilized efficiently to affect conditional changes?

Solution

One option is to split once from the right side, make the second part lowercase, then combine:

tmp = s.str.rsplit('-', 1)
out = tmp.str[0] + '-' + tmp.str[1].str.lower()

If the last part is always a single letter, @Barmar's solution is even better:

out = s.str[:-1] + s.str[-1].str.lower()

Output:

1    IX-1-a
2    IX-1-b
3    IX-1-c
4    IX-1-d