Checking for trailing letter

[jgelpi16]

I know there must be an easier way to do this, but this is the only way I currently know. I want to check to see if the 4th character in an account number is a letter. The string could be five or six characters, but the first three characters are always numbers. There is a possibility that the 4th character is a number, and if that is true I want to pass it through. This is what I have so far in my query, but this does not check if the 4th character is a letter or not, just if the string is greater than three characters....

IIf([OutstandingAR]![AcctNo]>Mid([OutstandingAR]![AcctNo],1,3), IIf([OutstandingAR]![Tax]>0.001, IIf([OutstandingAR]![Balance]>0.001,[OutstandingAR]![AcctNo],""),""),[OutstandingAR]![AcctNo])

Any suggestions are welcome. Thank you.

[jgelpi16]

Correction....I need to check to see if the last character is an Alpha character, not the 4th character.

[jgelpi16]

I used the expression:

 ReturnAlpha: IsNumeric([AcctNo])

and put "False" in the criteria field.