It just isn’t possible to know somebody through and through after having only dated them for 4 months. You want a perfect fairy tale “soul mate” kind of thing. There isn’t such a thing. Like @WasCy said, if you were truly “soul mates” (which I don’t think exists) then you should be able to wait decades to be together.
This whole soul mates thing just doesn’t hold water. There are other people (maybe a lot, maybe just a few), who if you met them and dated them for awhile would be a good fit for you too. That’s what you really want is someone who is a really good fit for you (not someone who is a perfect person or a perfect match, because there is no such thing).
At 4 months, you really can’t know if the other person is a good fit for you. You are caught up in the romantic, lovey-dovey feelings of young/new love. It is super-easy for young people to fall in love with all sorts of people who are not a good match. Hence the divorce rate.
You need to be together for a good long while. I would say even a year is not long enough. I think 5 years is a more realistic measurement. That way you will most likely have finished your schooling, started your careers and lived together (or at least in close proximity where you see each other every single day). During 5 years together you will hopefully have seen each other at the best and worst moments. You need to see the Good, the Bad and the Ugly to see if you can tolerate and even embrace all of it.
Your girfriend probably thinks you’re really cute when you pick her up and take her out on a date, but will she still think you’re cute when you’re clearing your throat and figuring out what to do with the contents of said throat?
You probably think your girlfriend is really cute when she giggles, but is she still that alluring when she’s making loud “pooting” sounds in the bathroom and uses up all the toilet paper?
What happens after the tenth time that one of you forgets to put a new roll of toilet paper on the spool or the tenth time that one of you forgets to mention that you need milk before the carton runs out, or the tenth time that one of you forgets to take out the trash?
I second the idea of going through a course of pre-marital counseling. The counselor will bring up all sorts of questions and scenarios for you to think about that are very important, but are usually not part of anyone’s conversation when they are just dating for 4 months.
You said that you and your girlfriend are of different faiths. That might not be a problem right this minute, but when/if you have kids, the faith/religion thing usually comes to the forefront. You said that she can do whatever she wants for Christmas, but what will you do if it becomes very important to her that you go to midnight mass with her and put up a tree and decorations all of your house, and prepare or eat certain holiday meals and dine with particular relatives and if she cries if you don’t go along with all of this?
Some questions that you should ask each other (and get good, true answers before you even consider marrying) are:
If she gets pregnant before you get married, what will you do? Will she keep the baby, have an abortion, give the baby up for adoption? Hand over sole custody to you? Give the child to her parents?
How many kids do each of you want? Would either one of you be disappointed if the kids were of the “wrong” sex (that you really wanted)? Would either of you be disappointed if you could not have kids of your own? Would you stay childless or try to adopt?
What kind of financial situations do you both have? Will you have joint accounts?
Do you have enough money to put a down payment on a house? If you end up having kids, do you have enough money to buy life insurance? Do you have enough money to put yourselves through college? Do you have enough money to pay for health insurance? Do you have enough money to buy and maintain a car (repairs, insurance, upkeep, gas)?
How will you divide up the household tasks? Who will be responsible for cooking, cleaning the house (vacuuming how often? cleaning the bathrooms how often and to what degree of cleanliness? Picking up after yourselves—laundry, food wrappers, paperwork), mowing the lawn, taking out the trash, washing the dishes (how often and does the kitchen need to be cleaned immediately?, emptying the dishwasher, doing the laundry, ironing, folding and putting away the clothes. To who’s standards will all of these tasks be done? Yours or hers? What happens when one of you fails to live up to your part of the housework bargain?
How often will you visit each other’s families and friends and how often or welcome will any of these people be in your home? Do you like these people?
How do each of you act when you are sick? Are you moody and mean and whiny? Do you need to be left alone? Are you willing to pick up the slack and kindly care for you sick spouse?
Have each of you seen each other in the morning when your hair is all greasy and you have bad breath and un-washed armpits? How’s that going to effect you if you have to see that day after day? How do you like the sound of farting and belching and other sights and sounds that are usually kept hidden during the first 4 months?
Ask yourselves all of these questions and more, then experience these things with the other person for a few years. Then if you still feel the same way, after the lovey-dovey feelings have worn off, then you’ve probably found yourself a good match. But not a soul mate.