What do you mean by "R1" field? Is this a shape data entry, a User defined entry, or some other?
Are the values 0 - 3, and A - D the only possible entry values, or are there other, non-acceptable values.
The basic formula structure would be something like =IF( strsame(prop.R1, OR(A,B,C,D)), "Is letter", IF(strsame(prop.R1, OR(0,1,2,3))," Is Number", "Invalid entry")).
Where Prop.R1, or however the user makes an entry, is first compared to the letters. If there's a match, it provides the letter notification, if not, it checks the numerical values and either prints the number notification if successful, or the invalid notification indicating there was no match at all.