Sub AddPrefixToCell()
Dim lastRow As Long
lastRow = Cells(Rows.Count, “B”).End(xlUp).Row

Dim i As Long
For i = 4 To lastRow
    Dim currentCell As Range
    Set currentCell = Range("B" & i)

    If Left(currentCell.Value, 4) = "aaaa" Then
        currentCell.Offset(0, -1).Value = "bbbbb" & currentCell.Value
    End If
Next i

End Sub