My girlfriend wants a break?

So we have been going out for 4 years now and living together for about 3 of those years. My gf suggested that we get separate apartments so that we can try to rekindle the flame by making dates and being more spontaneous. I agree...for the most part. But she also thinks we should be free to see other people while still dating and working on our relationship. The way she explains it, she doesn't want to miss out on anything or other possible connections. She says she wants to be with me and ultimately marry me but is only 85-90% sure of that and she wants to make sure it is not a mistake. I found out that she recently went back to a guys apartment and made out, just to come home to me and give me a kiss goodnight. When I found out I was livid. I don't understand why we cannot work things out without seeing other people. I just want the opportunity to rekindle the flame without interference from other guys because lets face it, if you are looking for excitement of course you will find it with something new. I feel like I am at a very unfair disadvantage. Any help will be great. I already know what most guys will say to this..."just dump her and move on". I know, we all know but 4 years is a long investment, especially when you really love someone.