The standard ticket Bern-Lucerne v.v. is always valid via Zofingen or Olten. It is stated on the ticket as accordingly. It is not valid via Entlebuch.
If you want to travel via Entlebuch with a Bern-Lucerne ticket valid via Zofingen or Olten (or with a Entlebuch ticket via Zofingen/Olten), you need a so-called change of route ticket in addition. You can get it from the ticket clerk at the station or from the train conductor on the train.
When travelling with an Entlebuch ticket via Zofingen/Olten, you will have to pay the difference of CHF 4.00. In the opposite case, there is no cash-back, however.