Osmo mobile 3 have all the same futures as 4 (got it with the app update so no difference here ) motor and magnetic clamp is the only difference except the price ofcourse
Main difference is 4 has a more powerful motor resulting in better stabilisation and is better at handling heavier devices not to mention the all the new features over the 3.
Think the main difference in 4 is the magnetic quick-release, would still choose 3 for VFM. https://store.dji.com/guides/dji-om-4-osmo-mobile-3-comparison/
Good for the price. Haven't got as much use out of mine because it's such a faff putting the phone in the gimbal and stabilising etc. OM4 uses a magnet and just clips on and off which is so much more handy.
I would get the 4 instead of the 3.