Your bowing arm can be considered a set of "hinges" - shoulder, elbow, wrist, even fingers.  I found that difficult as well to start with - and as described by other members on the forum, for the four strings, imagine the bow moving in four different planes.  The widest separation of the strings, both vertically and horizontally occurs close to the bridge, the gaps between them lessening as you move towards the fingerboard - I'm not suggesting "normal playing" close to the bridge, but it may be easier to separate the strings if you try that first.   Just as your fingers of the left hand get "accustomed" to where they should go for correct intonation, the muscles in your right arm, and all the "hinge points" involved will gradually "learn" where the four primary bowing planes are.   Of course, when you move on to INTENTIONALLY playing two strings at once (double stops or drones) you'll have seven different planes the bow needs to move in !

Other folks suggest "imagining a string between each string" - and for individual strings - that's the one to MISS, and for double stops, that's the one to hit !!!