The !R of Carlos IV came in two varieties: In the year 1789, the coin appeared with the bust of C III, not IV. In the following year, the 1R coin appeared with either the bust of C III or C IV. I'm unaware of any 1R from the year 1791. The 'regular issue' began in 1792 and continued through 1808. If the size has been determined as having a diameter of 20mm, it is a 1R.
The above information assumes the coin is from Mexico; otherwise, all bets are off.
Don....