IF the campaign is set up that way, you can get away with a low number of items in 4E. 3E, you could do that without having to resort to optional rules. The spells were there to add bonuses.
But, what if the 4E campaign is not set up that way? Then it is item reliant.
And what happens if the 25th level Fighter drops his +5 mega Sword into the lava in 4E? He might have an old +3 spare one, but he might not.
Like I pointed out, in 3E there were spell options to handle this even in a low magic setting.
It's either / or in 4E. Either one plays the normal rules with a lot of required items or one plays the inherent bonus rules.
A DM could theoretically pull in the inherent bonus rules for the 25th level Fighter if he so desired because the Fighter lost his sword, but that's fairly lame. A DM can do anything. That doesn't make the default system any less magic item reliant.
If you want a low(er) magic campaign you set up the campaign with low(er) magic rules.
Why would the 25th level Fighter drop his sword into lava? Last I checked, the only way for a character in 4e to drop his weapon is willingly. If the player is really dumb enough to drop his sword into lava of his own free will, oh well, sucks to be him. I guess he better hope the party ritualist has enough arcane components/residuum to whip him up a new blade right quick, or that he has a backup weapon. I don't think it's a very realistic scenario though.
A more realistic scenario is a rust monster. Except that the fighter isn't likely to be put out for more than that single encounter (and that's if the rust monster can actually land the attack) because you get full value residuum and it's child's play for the party ritualist to make you a new one as soon as the battle's done. Not exactly something I'd call game breaking.
It isn't realistic to say that in 3e the party could fall back onto spells. If it's a low/no magic campaign, the party wizard isn't exactly going to be lugging around a golfbag brimming with wands. And his spell slots are limited, so if he's memorizing enough Bull's Strengths, Greater Magic Weapons, etc. to buff the entire party (every fight for some of those if you're using the 3.5 rules, since 1 min/level is a pretty short duration) he isn't exactly going to be tossing web, dispel magic, or disintegrate like they're going out of style (and therefore isn't contributing much, if anything, to the encounters). And on top of that he
still can't compensate for the loss completely, since there were certain bonuses (saves) that spells in 3e generally don't grant. Oh, and it takes just one spell- (greater) dispel magic- to remove the party's "magic items" in this scenario. Hence, yes, the 3e party is at a noted disadvantage.
In order to compensate for all the enhancement bonuses you need, you'll be lugging around Merlin the Crappy Crossbowman with d4 HD and No Armor, and your bonuses will be worse than expected for your wealth level.
Heck, if the 3e fighter drops his +5 sword into the lava (which is far more likely to occur as 3.x actually allows disarming) he's pretty borked too. It isn't exactly likely that anyone would bother to memorize Greater Magic Weapon in a high level party that's practically drowning in magic bling. Better hope the fighter has a +4 backup, just like in your theoretical 4e scenario...