(vb)判断字符串是否为四位互不相同的正整数

添加了补充问题
Public Function GetStrNoSame(ByVal in_Str As String) As String

Dim i As Long, J As String, out_Str As String

For i = 1 To Len(in_Str)
J = Mid(in_Str, i, 1)
If InStr(1, out_Str, J, vbTextCompare) = 0 Then
out_Str = out_Str & J
End If
Next i
GetStrNoSame = out_Str
End Function

Private Sub Command1_Click()
Dim a As String
a = "1535"
If Len(GetStrNoSame(a)) = 4 and left(a,1)<>0 Then
MsgBox "a为四位互不相同的正整数"
Else
MsgBox "a不是四位互不相同的正整数"
End If
End Sub

一楼的正解,不过首位是零的情况不用考虑么?