Exactly, if your paying attention to everything around you you should have no problem at all, I have never missed an emergency coming from behind me while using my IEM's because I see them and even with out them on I see them way before I ever hear them.
Even if I have the sound system that I put in my old Civic I'd still rather listen to my IEM's because like so many car sound systems they're all about bass and they just don't sound good to me anymore. My friend that put a $4000 system in his car with really good tweeters and mid sized speaker and all he had was one 12 and it still was way to bassy for my liking. Take out the sub and it sounded much better but still not as good as the sound from a top-tier IEM imo.
Actually, car audionsystem can be very good also if the installer have a very good experience and know well how to install decent audio system in a car
I personally think, 4000 is not enough to install a DECENT car audio system
And yes, iem are much much cheaper and nice to use