The example macros below shows how you can fill a ListBox-control (and a ComboBox-control) in a UserForm with the unique values from worksheet range. Copy the code to the UserForm code module.

Private Sub UserForm_Initialize()
Dim MyUniqueList As Variant, i As Long
    With Me.ListBox1
        .Clear ' clear the listbox content
        MyUniqueList = UniqueItemList(Range("A4:A100"), True)
        For i = 1 To UBound(MyUniqueList)
            .AddItem MyUniqueList(i)
        Next i
        .ListIndex = 0 ' select the first item
    End With
End Sub


Private Function UniqueItemList(InputRange As Range, HorizontalList As Boolean) As Variant
Dim cl As Range, cUnique As New Collection, i As Long, uList() As Variant
    Application.Volatile
    On Error Resume Next
    For Each cl In InputRange
        If cl.Formula <> "" Then
            cUnique.Add cl.Value, CStr(cl.Value)
        End If
    Next cl
    UniqueItemList = ""
    If cUnique.Count > 0 Then
        ReDim uList(1 To cUnique.Count)
        For i = 1 To cUnique.Count
            uList(i) = cUnique(i)
        Next i
        UniqueItemList = uList
        If Not HorizontalList Then
            UniqueItemList = Application.WorksheetFunction.Transpose(UniqueItemList)
        End If
    End If
    On Error GoTo 0
End Function