Bros before hos my friend.
This may sound mean, but have you thought about the fact that she may be playing you both? She knew her boyfriend was leaving to somewhere far off right, so, for the time being she wants to use you as the "plaything" for roughly 8 months. Once the bf gets back, she'll do the old "Break up with you then tell the old bf that you pushed her into it ,yadda yadda, but I want you more card shtick."
It seems to me she's moving on a bit too fast for a 4 year relationship, and to me she sounds like she's one of those girls that just can't be alone. She sounds like the type that has to have a bf, either longterm or after a break up she's already with someone else a week later. Frankly, I dislike and distrust anyone like that(even males). If you can't be fine on your own, then how do you expect an SO to fill that void?
Although consider the source here, I am a 22 year old virgin that has a damn hard time talking to the opposite sex, but from what I've heard, this sounds like the motive above, and I've seen it happen one too many times in my life(to other people).
So, I say be cautious, if you truly think she likes you and isn't just using you for a boy toy till your friend gets back, then I say go for it, but if there is the slightest bit of doubt and you really like the friendship you have with the guy and value it. Then I say leave her the hell alone.