Can studs be 2-4 inches apart?
Yes.   They can be however far apart the builder wants them.   I have used 3-4 studs within 16&quot; when doing bathrooms, high traffic corner, in bathrooms for showers... and have installed an extra stud because of a previous mistake.
I only use magnetic stud finder (finds your screws).
If you use a method and you just aren't 100% sure the best thing to do is a test with a monkeyhook.   These take literally 10 seconds to test with no equipment and only leave a tiny pinhole.   If you hit something... boom got a stud.   If you don't no one will notice (you can cover the pinholes with similar colored toothpaste!).   Also monkeyhooks are great for holding smaller picture frames.
Also adding based on comments on plumbing:

Paramagnetic metals are weakly attracted to magnets, and include
platinum, tungsten, aluminum and magnesium. Ferrimagnetic metals like
magnetite are also attracted to magnets, while diamagnetic metals like
silver and copper are repelled by them.

Your magnet will slightly repel (and have tested) on copper (obviously stud finder or magnetic stud finder will not find pex).
To add specifically to mounting:  You can remove drywall and add cross bracing.   You are basically ruining that small section anyway.   If you want to beef it up - it takes 30 mins.