strVar = InputBox("Enter the Key")
Set objReg = New RegExp
objReg.Pattern="^[a-zA-Z][0-9]{3}$"      'If you want only the capital letters, you can rewrite the pattern as ^[A-Z][0-9]{3}$
If Not objReg.Test(strVar) Then
    MsgBox "Input is incorrect!!!"
Else
    MsgBox "Input is correct"
End If
Set objReg = Nothing