It seems the common advice is you are doing the right thing by waiting but I'm going to suggest something different.  Yes, by all means put it off until after these critical times have passed for her, that is just being considerate but I'm also going to suggest that you spend all the time you are together between now and then trying to ensure you are making the right decision.  She could be a little off because of the stress she is under so try not to be judgmental and see if you can't make a list of reasons it was not meant to be, then consider if these aspects could be related to stress. I think it is really nice of you to postpone this until she gets past the exams, but also make sure you aren't throwing away something that could be good here. Keep in mind you are also throwing away four years you have already been together and I find it strange it would take you over four years to figure out that it isn't going to work.

So try cutting her some slack and do some nice things for her in this time of high stress, see if she responds to a little extra TLC. Even get one of those vibrating foot baths, fill it with warm water, bring it to her, complete with a towel and extension cord of sufficient length and put her feet in it at the end of the day. After she has had enough of that, dry her feet off, throw the towel in the laundry and put the foot bath away and clean up a bit while she has the chance to relax.

Find some simple, romantic and thoughtful things you can do for her (like the foot bath example above) and see where it gets the both of you.

If it doesn't work out in the end then it doesn't work out but at least you will know that you even put in some extra effort to try to make it better and it still didn't work so you need not lament over it after that,  Good luck!