4000 satellites is enough to hit economies of scale. Per piece they are probably among the cheapest satellites in orbit.

Sure, but that isn't saying an awful lot; most satellites are very expensive. SpaceX would want to weigh the cost of hardening the system against radiation versus the cost of another launch to replace failure(s). I'd guess radiation-hardening of some kind (at least sufficient to prevent most permanent damage) would win out in that calculation, but I'll happily admit I don't have any particular factual basis for that guess.