Marriage is a big deal, and I think it's worth considering the worst case scenario. Every person in my family has been divorced at least once, and it was hell for all of them. I think you should hesitate before putting yourself through that.
Your finances will be intermingled with this man's. You will be responsible for his debt, and your credit score will be attached to his. How good is he with money? Would you let him borrow money if he asked? How much? If you wouldn't even do that, why are you comfortable linking your financial future to his? How will you decide on a family budget?
You will be living with this man. What are his home habits like? Does he clean up after himself? What arrangements will you make for cooking? Who will be responsible for trash, or shopping, or cleaning up after the family pet? None of these are insurmountable problems, but if he has a series of bad housekeeping habits you're not aware of (since you've never lived with him), you may quickly find a deluge of things you can't tolerate, and you'll need a divorce.
You may have children with this man. What are his thoughts on parenting? How does he feel about corporal punishment? How do you feel? Are there religious differences? Who will be responsible for raising the children? If you, are you willing to give up your career? If him, is he?
If you do get divorced, things can be ugly. You will need to work out how your property will be divided. Lawyers will be milking both of you for all the money they can. If you disagree, you may both come to despise the person you once loved. If there are kids involved, they might get used as bargaining chips, or made to feel like they're the cause of the problem, emotionally damaging them for the rest of their lives. If there is alimony or child support, you may wind up fighting each other in court for decades over money.
Marriage is a wonderful thing, but it can be a terrible thing when it's not to the right person. Do you feel confident that after 4 months you can answer these questions? If so, then go for it. You're better prepared than most. I doubt that's the case though, and I'd suggest against accepting the proposal for a while longer. Let him know that it's not for lack of love - it is for these specific questions. You're attracted to him and you love him. You need to get to know him better before you commit to spend the rest of your life with him.