My husband and I are raising a quadrilingual daughter who is currently 5 y.o. and equally proficient in all 4 languages. We were aiming for trilingual actually (my native Romanian, his native German, and Swedish once she'd start preschool), but around 2 years of age she started showing clear signs that she understood English as well - which is the language my husband and I use with each other.
In short: we have been using the one person one language method (as described by Stan above) from birth. Even now at the age of 5 we still stick to alternating bedtime reading evenings, which of course guarantees equal exposure to extra vocabulary outside the everyday spectrum of conversational topics. In the first year of life when I was on maternity leave she was obviously exposed more to Romanian, and we tried to balance that by dad taking over as much as possible during the weekends (extra reading, walks with her in a wrap so he would constantly talk to her, skyping with his family, etc).
Also during the first year some exposure to Swedish started since me and baby were frequent visitors of different baby groups where there was singing, reading, or just chatting to other parents in Swedish (association with activities). She then started preschool at 18 m.o., in Swedish. Note: no Swedish at home! (association with places) Home was (and still is) exclusively for "our" languages, although she may choose today to occasionally watch some cartoon in Swedish (we want to leave that choice open for her).
Once or twice a year we travel to the grandparents without the other parent, for her to get full immersion into one language only (though we haven't done that last year because of the pandemic). These trips usually give a tremendous boost!
As for English: it simply happened by "passive" exposure, initially. We did not direct English at her at all until after the age of 3, once it became clear to us that she had identified English to be our "inclusiveness language". For instance, we would sit around the dinner table, we'd be carrying out some simple parent-child conversations in our respective languages, and she would try to reply in English instead so the other parent would also understand. This started happening around 2-3 years of age.
I recommend trying to find kids of the same age who are natives in the respective languages and meet regularly. As kids grow they will enjoy finding use in all those languages outside of home, best if associated with playing.
Today: English is our family language - we use it when all of use are part of a conversation, to watch some animation together, etc. Otherwise it's Romanian or German. Occasionally she may use Swedish when she wants to reproduce some dialogue from preschool. Cultural life outside the home is in Swedish (theatre or music shows). We estimate that 50% of her awake time is in Swedish, 15% Romanian, 15% German, and 20% English. By this rate, we expect Swedish to become more and more dominant once she starts school and will be able to read and write in Swedish and enrich her vocabulary by studying sciences, maths, history, geography, etc. We will keep doing our best at home and provide her as much exposure to the other languages as we can and for as long as she will enjoy it!
I hope this helps and good luck. It'll be an amazing journey!