Originally Posted by Ostinato
I don't really see what the problem is, because you've not done anything wrong.
During your 4 year relationship, if this is honestly the first time either of you have had attention from other people then I'd be surprised - as it's actually quite common for partners to get interest from others.
You should just say to this girl your with Kels and you love her, and that no matter what she has no chance with you so she should stop wasting her time, then if she continues speak to your girlfriend and explain to her you think she's been a bit flirty, but choose your words wisely. Think about it, if your all like 'THIS GURL THINKS IM FITT!' she'll be quite jealous and maybe a bit worried. You need to reassure her you have no interest in the girl and that shes the only one for you.
However, if you actually like the girl... well - that's a whole different kettle of fish! :p