Yes, sort of. There are no rules for it, but you can do so anyway. However... you probably shouldn't, which is probably why there aren't rules for it.
D&D 4e has these things called treasure parcels, which you've probably heard of if you've opened the Dungeon Master's Guide. They advise the DM in the quantity of magic items and wealth which should drop each level. Sticking to these numbers is hugely important for D&D 4e, since the mathematics the system is built upon rely on you getting more or less exactly that stuff.
This is very different to how previous editions of D&D worked.
So you could loot those enemies for their equipment. However, to keep things within the bounds the game expects, if you're collecting helmets just to sell them to NPCs, the DM should treat those helmets as a portion of the total monetary treasure you're going to get from your treasure parcel. It's probably more useful to get that monetary treasure in the form of actual money or small (but valuable) gemstones than in the form of a hundred helmets stuffed into your packs.
Pinching magic weapons and armor and potions, however, is probably fair game, since if you can loot that stuff to begin with, that's probably your DM's method of handing you some of your magic items and potions from your treasure parcels for that level.