Well, let's play the theoretical scenario game (since I haven't seen anyone do it here yet, or I missed it).
Let's say Lauren keeps her idol and uses it at F4, and assume everything still shakes out the same way (Rick wins immunity and gives his idol to Gavin w/ a F4 promise), the only person not immune so is eliminated is Julie.
If Chris wins F4 immunity then nothing really changes and he still wins. But if Lauren won F4 immunity then Chris doesn't get his nice little moment, she would probably take him and have Gavin and Rick battle it out.
If Rick wins that it's all over, but if Gavin won, we'd have a final 3 tribal of Lauren, fire-winner Gavin and no-immunity-or-fire-making Chris; surely Chris doesn't win under that scenario? But then who wins, Lauren or Gavin? Eh, probably Gavin but it'd be closer...
I think you mean F5, given you can't use idols at F4. ---