1. Marriage talk after 4 months of dating is far too soon. These people are obviously very young and inexperienced. IMO you should date 1 year before even talking about marriage. Talk about other long-term issues, but don't make wedding plans. I also think both people should be at least age 25. Statistically, the younger you marry, the more likely you are to get a divorce because you didn't really understand what you wanted in a partner. (I married at age 23 and got divorced later. NOW I know what I need in a partner.)
2. Michelle doesn't know what she wants which is why she is flip-flopping between guys. She needs to grow up. She is just bad news for the guys.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)