Try this

Code:
Private Sub CommandButton1_Click()

    Dim lngColumnIndex() As Long
    Dim lngLoop As Long, lngSelected As Long, lngRows As Long
    Dim strColumnHeaders As String, strSelected As String
    Dim blnHoldsTrue As Boolean
    Const lngColumnHeaderRow As Long = 1
    Const lngColumnsToCompare As Long = 4
    
    On Error GoTo Err
    ReDim lngColumnIndex(1 To lngColumnsToCompare + 1)
    For lngLoop = 1 To ActiveSheet.UsedRange.Columns.Count
        strColumnHeaders = strColumnHeaders & lngLoop & " - " & Cells(lngColumnHeaderRow, lngLoop).Value & "|"
    Next lngLoop
    strColumnHeaders = "The column headers are " & vbLf & vbLf & Join(Split(strColumnHeaders, "|"), vbLf) & vbLf
    For lngLoop = 1 To lngColumnsToCompare
        For lngSelected = 1 To lngLoop - 1
            strSelected = strSelected & lngColumnIndex(lngSelected) & vbLf
        Next lngSelected
        lngColumnIndex(lngLoop) = InputBox(strColumnHeaders & strSelected & "Enter each column index one by one", "Column Compare")
        strSelected = "You have already selected:" & vbLf & vbLf
    Next lngLoop
    
    For lngSelected = 1 To lngLoop - 1
        strSelected = strSelected & lngColumnIndex(lngSelected) & vbLf
    Next lngSelected
    lngColumnIndex(lngLoop) = InputBox(strColumnHeaders & strSelected & "Enter column index where you want to show the comparison result", "Column Compare")
    lngRows = ActiveSheet.Cells(Rows.Count, 1).End(xlUp).Row
    blnHoldsTrue = True
    For lngRows = lngColumnHeaderRow + 1 To lngRows
        For lngLoop = 2 To lngColumnsToCompare
            blnHoldsTrue = blnHoldsTrue And (Cells(lngRows, lngColumnIndex(lngLoop)).Value = Cells(lngRows, lngColumnIndex(lngLoop - 1)).Value)
        Next lngLoop
        Cells(lngRows, lngColumnIndex(lngLoop)).Value = blnHoldsTrue
        blnHoldsTrue = True
    Next lngRows
    Exit Sub
Err:    MsgBox "Either cancelled by user, or incorrect entry made." & vbLf & vbLf & "If neither of these, unexpected error!", vbOKOnly + vbInformation, ""
    
End Sub