You are not married yet. You have not tied your financial life, social life and emotional life to her in the same way that comes with marriage. You can't trust her now when she should be on her best behavior. How can you trust her with your life? Can you imagine trusting her with childrens lives - your children? Believe me, this will end badly. Do not let a 4-1/2 year investment of your time keep you in a shitty place. You did not waste your time! You learned a lot about relationships. You just learned an important lesson - you can't trust a cheater. Don't take the next course: "Once a cheater, always a cheater!" I will give you the quick lesson, she cheated on you because it is within her personality make up to lie, cheat and betray you. It is WHO she IS! It will not change. I have known a lot of cheaters in my life, that is who and what they are. Don't do this to yourself and more importantly, don't do this to your future kids. They don't want a mommy who is a whore and winds up divorced!

Ok, so you said she kissed him and "went down on him". Does that mean she gave him a bj?  Makes a big diff in my mind. Did she suck him to completion? Swallow his cum?  A kiss is kiss. Sucking him off is unacceptable. Especially if she took his load. So what will you be thinking about every time you kiss her from here on out? Did you ask her why she did it?  What makes you think she will not do it again sometime down the line and really screw you up?  Not worth the risk.  At least tell her you need a break.  If she loves you as much as you think she'll be around after a month or so.

You need to determine a couple of things:

1) she needs to answer 'why' she did it. Like, why did she give herself permission? Why did she do it, aside from how she felt? Get an honest answer and not something generic and dramatic. See if you believe her.

2) understand that if you stay with her, you have to accept that this comes with it. This incident will be in your memory forever, and it will impact your level of trust. She will have to work long and hard to repair your trust, and it is possible, but your trust will be more fragile thereafter.

3) she obsessed over another guy. Either she listed after him more than you because he was that more appealing than you, or because she gets this way with guys every so often. Either way, your position as 'one and only' in her mind was compromised. Now, there is one guy, or perhaps some future guys, who have and may again take up that spot in her head. It happened, so.. can you live with that? 

Going on 4 years... google sunk cost fallacy. 

As people before said, why did she kiss and suck another guy off? The quick answer is that he provides something you do not. That is always going to be a problem, so yes, she will do this again.  (Plus, maybe it was more than 1 in the past.) Though, I am biased by the American Pie theory, rule of 3. Divide a guy's number by 3 and multiply a girl's number by 3.

Just move on. Plenty of nice women out there and girls like her for the interim time until you meet that nice woman.

did she swallow his cum ?