It comes down to reasonable threat--it doesn't really exist to be reasonable enough to just say it's plain unsafe and that's that. One extreme isn't better than the other--can't say it's completely safe, but it's not doing much good to say it's just unsafe (and imply it's basically essentially useless in that respect because of it).
It's dangerous, period. It's irresponsible to tell others to use it.
As for 3GS and 4, the 3GS uses a 600 MHz Cortex-A8 (perhaps downclocked from 800 MHz, while a 4 uses 1 GHz Apple A4 (even it might be downclocked to 800 MHz or slightly lower). Although Apple A4 is based on Cortex-A8 it's still not the same thing.
They both use A8s, one's 600Mhz, the other's 800Mhz, as I recall.