This is one of those statements like "4e is deadlier than 3e" that is completely untrue on its face. 4e's game design is one in which characters:
• Heal to full with six hours of rest.
• Survive several hits before dropping.
• Usually heal to full between encounters.
• Have character classes that heal by shouting encouragement.
• Will almost always survive a critical hit.
• Start as "heroes" from the day they are created.
The only way that 4e is less gritty is when DMs hand out wands of cure light wounds like candy (which hasn't happened in any 3e/Pathfinder game I've ever run in). 4e is designed to be less gritty straight out of the box.
Based on my many years of playing both 3E and 4E your argument that 3E is a default grittier game than 4E is false. All of these bullet points presented that supposedly make 4E a less grittier game exist in 3e as well. (Assuming a standard party of cleric, wizard, rogue, fighter). The truth is both games are equally "non gritty" games if played by the default rules.
In 3E Every time the party rests for the night they heal to full... because the cleric converts every spell he didn't cast into a healing spell and sprinkles it on to the party one by one until they are all cured. If for whatever reason the cleric runs out of spells due to a grueling day, then they break out the CLW wand. Every game of standard 3E I've ever played has seen the party heal to full after a night's rest. In 4E every time a party takes an extended rest this process is hand waved and everyone simply heals to full. The effect is the same in both systems.
Every 3E character I've ever played has been able to survive more than one hit before dropping (with one exception see below). First level is the only time a character remotely has a chance to be one shot. And the first level wizard I played I assigned con as his secondary best score and gave him the toughness feat, so he started off with enough plot protection to keep him safe as well.
So a 4E party has a warlord instead of a cleric, maybe one group hates the flavor maybe another likes it... but bottom line is both editions have about the same ratio of healers to damage dealers in the makeup of the party, so both editions have the same amount of healing going on.
Other than a game where a first level character got obliterated by an orc with a greataxe crit, I have yet in my years of play experience to see a 3E player character taken from full hp to -10 hp from a monster crit.
There is a reason 3E characters start out with max hp, higher ability score arrays, and access to special PC character classes... because from the time they begin the game they are heroes. NPCs often use classes like commoner, expert, and warrior that are suboptimal for a reason.
DMs that play 3E by the rules don't pass out cure light wounds wands like candy. DMs that play 3E by the rules however, allow a party member to take the "craft wands" feat and create a 50 charge cure light wounds wand for 375gp and very nominal xp. (Or better yet a wand of lesser vigor).
Sure, I've played in house ruled 3E games that disallow CLW wands but those are house ruled games. A house ruled game to make the game more gritty. There are plenty of house rules one can introduce to the default 4E game to make the game more gritty as well. I suppose there are games out there where the players intentionally choose not to equip themselves with CLW or lesser vigor wands, even if the DM would otherwise allow it, but I haven't played in any.
Bottom line is the standard 3E and 4E rulesets are "non-gritty" systems. I'll buy into arguments that BECMI or AD&D is a more gritty system, and there are some grittier settings such as Ravenloft or Dark Sun, but Vanilla D&D as a whole compared to other fantasy RPGs tends to be less gritty unless the DM takes steps to home brew a game world with house rules to intentionally make the game grittier.
