Take it as a compliment. What she means is that she can envision a future where you and her end up as a happily married couple some day. It doesn't mean you need to get engaged or make any promises any time soon. In fact, at four months, she might find it a little creepy if you did propose. You still have things to learn about each other and more growing to do to together as a couple.
What you can start doing right now is to give some thought as to whether you feel the same way as she does. First of, do you even plan to get married some day? If so, does your girlfriend seem like the type of woman you'd like to end up married to, or is she just someone to have fun with? Make sure you give some thought to whether she seems like she'd be a reliable and responsible partner because those things are going to matter if you end up marrying her. If you really can't see yourself marrying her, then you should let her know. Otherwise, she's like to feel as though you were misleading her by not saying anything. On the other hand, if you can picture yourself marrying her someday, you can discuss that with her as well.
You can also take this time to start thinking and talking about when would be a good time to be engaged and married. How long do you think you need to date? How old would you like to be? Do you need to be financially secure first? What other things would you like to accomplish before you get to that point? What are things you want to learn about each other or questions you want to ask before you get married? I'd recommend making sure you've dated at least a year before getting engaged and at least two years before getting married. At least one study found that if you date for two years before getting married, your marriage has a much better chance of success than if you date for under two years.
The mistake a lot of people make when they're dating is viewing marriage as an all or nothing thing. They either think that if you like the idea of marrying somebody, then there's no reason not to get married straight away. Then they often rush into marriage only to find out afterward that being married to their partner isn't what they thought it would be and want a divorce. Or they recognize that it's too soon to make the commitment, so they don't want to discuss or think about it at all. Those couples often end up years later with one partner eagerly awaiting marriage and the other partner not feeling ready at all. That can cause a lot of conflict and problems in the relationship as well.
You can avoid both problems by recognizing that there are different levels of commitment. Commitment should be something that grows as you learn more about each and prove yourself to each other each day. As concerns or issues come up, you see whether they can be worked through or not. Then when you've had a long enough period of time of feeling confident about your partner and your relationship, you can move toward more serious levels of commitment. By recognizing and acknowledging that it's what you're doing, you'll save yourself a lot of hassle down the line. Good luck!
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy