This works

Private Sub txtbxPhonePost_Exit(ByVal Cancel As MSForms.ReturnBoolean)
If txtbxPhonePost.Value = "" Then
usfDemo.cmbxDOBMonth.SetFocus
Else
If IsNumeric(usfDemo.txtbxPhonePost.Text) = False Then
MsgBox "This box must be blank or contain exactly 4 numbers!",
vbOKOnly
Else
If Len(usfDemo.txtbxPhonePost.Text) < 4 Then
MsgBox "This box must be blank or contain exactly 4
numbers!", vbOKOnly
End If
End If
End If
End Sub

Is there a better way?
--
Thanks
Shawn