I don't think I agree with this. The question is not whether the DM can choose powerful, PC-killing monsters like Irontooth in 4E (who, by the way, is nothing but your typical level 1 goblin); it's whether typical fights allow a high-survivability rate for PCs in 4E vs previous editions, if the DM so wishes.
Personally, I think that 4E allows for the possibility of higher survival rates at low level.
The reason is that the lowest level monsters (level 1) in 4E don't deal enough damage to put a PC unconscious in one shot. Most PCs start with 20+ HPs. Recommended normal damage expressions for level 1 monsters don't do 20+ damage. Thus, it's likely that a PC will remain standing after one attack in 4E.
In 3E and before, however, first level PCs could easily be downed with a single stroke by weak opponents that dealt 1d6, 1d8 or 1d10 damage plus bonuses. With a mage that had 1d4 (+ possible small bonus) hit points, or a rogue that had 1d6 (+ possible small bonus) hit points, chances of going down in one shot was present; chances of going down in one battle was high. 3E alleviated this partly by providing max HPs at level 1, but still, starting out with 6 HPs and meeting a basic orc warrior with a basic long sword was stressful.
So in previous editions, even if the DM didn't want to kill the PCs, it was a bit harder because your basic, run-of-the-mill opponent could well down a PC with a single basic attack.