first of all- congrats, Ben!
Any woman worth her salt as a wife would slap you for suggesting that you cash out even part of your 401 for a ring/wedding. Your heart is in a good place, and its a nice gesture to be generous about the ring, but really think things over. We're still young, but now's the time to be contributing as much as you can to ALL retirement options, simply because of the interest. Seriously, read up on this and please look for another option. Living below your means in your 20s is the best gift you could give you, your wife, and any potential kids (or potential car projects). A ring is a depreciating asset, no matter what the industry wants you to think. Maybe look at craigslist or a pawn shop? I hear there are absolute steals right now, because 99% of the time, you cannot get back the money you pay for a new diamond/setting.
Also, have you had any type of conversation with her about it? Maybe she doesn't want a diamond. Are you planning on talking to her mom/dad/grandma whoever raised her? Some women come along with an heirloom.
I know the ring seems important to women, but tbh I asked for a hose clamp. I would try them on at shops and Greg would : facepalm : and drag me away from them. When its the right person, the 'stuff' doesn't matter as much. Just keep that in mind if you do talk to her first. Best of luck, and please buy what you can pay cash for (or 0% finance, I didn't know you could do that). Its just not important enough to seriously stretch money for.