Because the 3rd-level Sor/Wiz spell girallon's blessing [trans] (Spell Compendium 106), in part, says that the spell grants "the subject an additional pair of arms. Each of its arms—new and old—ends in a clawed hand with fingers and an opposable thumb," and later says that the subject "cannot use normal weapons and the claw attacks in the same round, and the subject does not gain additional claw attacks from a high base attack bonus," this GM would rule that the extra pair of arms granted by the spell can wield weapons.
Since each of a human-shaped subject's normal hands "ends in a clawed hand with fingers and an opposable thumb," my opinion is that the game wouldn't bother saying that the subject couldn't make claw attacks if it makes normal weapon attacks and vice versa unless the subject could, in fact, make normal weapon attacks while the spell continues.
So, technically, if all four modified hands are wielding weapons, the subject picks each turn to attack either with its normal (presumably manufactured) weapons and its other natural weapons except for the claws granted by the spell or with the claws from the spell and its other natural weapons.
(Pathfinder includes a note for natural attacks when a limb is otherwise occupied: "[O]ften a creature must forgo one natural attack for each weapon clutched in that limb." Dungeons & Dragons 3.5 includes no such note. Still, as the girallon's blessing spell is arbitrary even for magic, this DM might borrow from Pathfinder and make the spell less weird by making a house rule that says that attacking with a weapon in a spell-modified hand reduces the number of spell-granted natural attacks that the creature can make this turn by one cumulatively.)
Wielding a bunch of weapons is easy; getting more arms with hands is hard
The girallon's blessing spell gives the subject four natural attacks. This is a big deal, and I imagine this is why many folks cast the spell: for example, a Bbn5 with Str 18 that's the subject of the spell potentially, all told, deals 6d4+22 points of damage (average 37) against a lone foe with the subject's spell-granted claws and rend to where he'd only deal 1d12+6 points of damage to that foe with a greataxe. At lesser degrees of optimization, potentially more than tripling the barbarian's damage output temporarily is awesome for a 3rd-level spell, especially since the spell's duration (10 min./level) means the spell'll likely last at least two encounters. By the same token, at high levels, the spell will usually be obsoleted by the subject's full attack.
Wielding weapons in the extra arms actually does very little as wielding more weapons doesn't in and of itself grant more attacks. Really, if you Dumpster-dive for them, your human-shaped creature can already wield upward of a dozen weapons simultaneously without any extra arms at all if that's your jam. The only real advantage to wielding a lot of weapons is being able to pick on a per-attack basis to make with one of those weapons that attack according to your base attack bonus and feat selections. (For example, a creature with a base attack bonus of +6 that wields 4 weapons makes one attack with a +6 base attack bonus and a second attack with a +1 base attack bonus, but each attack can be made with any one of the four weapons the creature wields.)
However, at greater degrees of optimization, a creature that's the subject of a girallon's blessing spell is technically eligible for the feats Multiweapon Fighting (Monster Manual 304) et al., and actually acquiring those feats during the spell's duration isn't impossible. (The DCFS takes 2 standard actions but is sometimes considered shady; the psionic power psychic reformation takes 10 min. but usually isn't yet in this case is harder to use.) However, while the feat Multiweapon Fighting "replaces the Two-Weapon Fighting feat for creatures with more than two arms," the creature won't actually benefit from that feat while it's not the subject of the girallon's blessing spell. (Seriously: If the subject could benefit from the Multiweapon feats without the spell then why would the subject take the feats in this nonstandard way?) Thus if the creature has a reliable source of the spell—plus, ideally, a source of extra damage like sneak attack—, then the spell can lead to a decently optimized lifestyle choice for an otherwise relatively mundane creature.
As always, if high optimization is the goal, measure your PC's performance by the threats the DM sets forth and the other PCs' capabilities. If you're killing one level-appropriate foe per full attack, then the party's other warriors should be also.
Note: That the spell allows wielding weapons in the new arms with hands is consistent with answers here. For more on the girallon's blessing spell's legacy text from its original appearance in Savage Species, see here. For more on the weirdness of Multiweapon Fighting with regard to Pathfinder but applicable to D&D 3.5, see here. For more on wielding two or more two-handed weapons with four or more arms with hands, see here. For more on wielding three-or-more-handed weapons in three or more arms with hands, see here.