(I actually don't know how people can survive without one)
I agree.
It's telling that a good 75% of the threads on the Sleep board are from babies without a dummy. Close to 99% of the over 12 month old poor sleeper threads are babies without a dummy.
once she's in a deep sleep she spits it...
That's what is supposed to happen. Once in a deep sleep, babies jaw muscles relax and the dummy drops. The dummy is for getting to sleep, not staying asleep.
If you have a light sleeper who wakes often, that's not to do with the dummy. It's a seperate issue (which the dummy will make a lot easier to deal with).
The way to have a deep sleeper (rather than a light sleeper who wakes often) is simply having lots of sleep. The more sleep baby gets, the easier and deeper they sleep. So make sleeping as easy as possible.
...then im worried that once 4months comes we will have to get up constantly because she won't be able to get back to sleep without it
You need to reset your expectations.
Yes, baby WILL need you to put their dummy back in when she wakes. She won't develop the manual dexterity skills to do that herself until 7-10 months (child specific, they all develop motor skills at different rates).
So yep, "dummy runs" (going to baby just to plug dummy back in) are a thing. You cannot change that. The best you can do is to try for a baby who sleeps deeply and doesn't wake often. Then you have fewer dumny runs. The way to do that is to make sleeping easy for baby and get as much sleep as possible. So not swaddling when baby like a being swaddled wont help there.