Livio,
Roman numeral - Rome conducted the Crusades.
Arabic numeral - Rome sought to overthrow Muslim rule in Jerusalem.
Formal English numeration - The English made some contributions to help Rome fight the Crusades.
It could also be asserted that France, Italy, Germany, and other European countries were also involved.
But "Fourth" agrees with "Crusade" because both words are in English. However, you're right, it is a bit "cute". :D
Erica seems agreeable to using the Arabic numeral that you began with: "4th".
And I would find any of these choices equally suitable.
But you mentioned that WWI and WWII are denoted with Roman numerals, and that's a good point. So it should be alright to number the Crusades in that manner. Why not?
Except that IV means 4 or four. How do you say fourth or 4th in Roman numerals?
If it can't be done, you could write "Leader of the 4th Crusade" in the Label field and change Occupation to "Leader of the Fourth Crusade".
That covers at least two of the options for Google searching purposes.