game keeps crashing when spawning 4+ cars or using traffic

It's unclear:
It's crashing while it's spawning the vehicle, or after some minutes it has finished spawned them?

If you have not yet, check in Safe Mode first.

Also, note that spawning many vehicles usually takes a bit of time.
Consider around 5 to 10 seconds per vehicle (heavily depending on your PC).
If you have set Traffic amount to a high number, your game will freeze for longer while it's loading the vehicle.

If you start pressing buttons / clicking things, Windows will say the game is 'Not Responding', while it's actually loading the vehicles in background, as the message you are getting is saying.
Avoid doing that, and just give the game a bit more time. Again, this heavily depends on your PC. If you have mods, and Traffic could load them, this could take longer.

 

Like, how long after? A few seconds, some minutes? Some more details would be helpful
Have you yet tried in Safe Mode?

 

Replicate the issue and give a Support ID:
https://supportid.beamng.com/