If number in column duplicated on sheet change then problen

May be this,,,?

Private Sub Worksheet_Change(ByVal Target As Range)
Dim EvalRange As Range
    
Set EvalRange = Range("a4:a10000")

If Intersect(Target, EvalRange) Is Nothing Then Exit Sub
If Target.Cells.Count > 1 Then Exit Sub
If Target.Value = "" Then Exit Sub

Target.NumberFormat = "0000"

If WorksheetFunction.CountIf(EvalRange, Target.Value) > 1 Then
    MsgBox Target.Value & " has already been used." & vbCr & "Do you want to use the same number (cancel) or enter a new one (retry)?", vbInformation, "Value Already Exist"
    
    Application.EnableEvents = False
        Target.Value = ""
    Application.EnableEvents = True
End If

End Sub