I'm impressed by how reasoned and measured the responses have been to this thread. Here's my take, for what it's worth.
If it happened 4 years ago, and she's been faithful since, and you love her, and she loves you, I would advise you to forgive her and move on.
I think you're probably assuming that you would never, ever consider cheating on her. But I think one day you might very well find this assumption to be sorely tested.
I personally have mixed feelings about monogamy. The older I get, the more I come to conclude that monogamy is for many people an unreasonable demand/expectation. Some people are well-suited for monogamy, but others, both men and women, are not. Similarly, some people have very strong sex drives, others not so strong. To me, demanding that someone who is not well-suited to monogamy to be monogamous his/her own life is not too different from demanding that someone with same sex attractions refrain from acting on those attractions. (It should be clear from this that I see nothing inherently morally wrong with sex outside of marriage. I don't think sex all that big of a deal to merit so much moral angst.) There is only so much 'hypocrisy' and moral failing around sex in our culture because our culture has chosen to make such a big deal about it and adopt such a rigid religious-centric view of it--and marriage.
I believe monogamy to be a mostly cultural construct and not an 'eternal' or 'natural' moral truth. It has not been the norm for as long as humans have existed--at least for men (although presumably they've been having sex with someone and not all of them are gay), and certainly has not been the norm among the privileged classes or among the powerful.
Also, let's face it. Many people are not getting their emotional, psychological, and sexual needs met by their spouses, but are not at the same time willing to give up their children or their financial security or their marriage. Human relationships are complex and full of morally gray area, and over time, I find the black and white moral perspectives on these and other issues offered by a sexually uptight religious world view to be less and less satisfying, morally, intellectually, and just about every other way.