Sounds like you bought the correct string. It is the name and octave of the string that is important, not the number. 4th octave A is the same string position on all harps. What may vary is the type of string depending on the type of harp (pedal harp or lever harp, antique or modern, etc.). Troubadour harps take the same kind of string as pedal harps so you are okay there.
See this chart from Lyon & Healy:
https://www.harp.com/string-products/troubadour-iv/?fwp_finder_string_source=original-manufacturer-strings&brand=
Please note that nylon strings can take a long time to stretch out and hold pitch so keep tuning often. If there are enough coils on the tuning pin to reach the neck, you may need to unwind the string and pull it through the pin again to take up the slack, then rewind. Good luck and enjoy your harp!