Multiple Comboboxes In A Userform

I have a worksheet "Master Log" with a UserForm "UserForm2" set up that has 4 comboboxes and 2 text entries. I already have the filtered unique values for each combobox sent to columns "O" thru "R" and they are dynamic ranges. I have the code to populate one of the comboboxes in the userform but cant figure out how to modify the code to have all four populated without getting ambiguous entry errors. And also, do I have to initiate the form for each combobox? Below is the code Im using for a single combobox.

Private Sub UserForm_Initialize()
Dim MyUniqueList As Variant, i As Long
With Me.ListBox1
.Clear ' clear the listbox content
MyUniqueList = UniqueItemList(Range("o4:o100"), 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