I think you are making a big deal out of not much. Does your partner's faithfulness for the last four years mean nothing?
BTW - flirting does not necessarily imply intent, and a desire to be attractive to others is pretty universal. How did you come to find out about your partner's so-called lack of good judgement?
Based solely on the info you provided, you sound like a relatively insecure person. If that is an accurate description of you, you might want to work on that before you drive this person away.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?