OK. Assume four functional limbs with the lower set able only to carry off hand weapons and light shields.
Weapon loadout: Executioner's Axe, Handaxe, Light shield. Build: Marauder Ranger.
Did I just hear the phrase "Throw and Stab" - or Twin strike when one of the attacks is an executioner's axe? And with a decent AC when you combine a light shield with Two Weapon Defence.
Weapon loadout: Glaive, light shield, dagger. Build: Polearm Fighter
You just gave Tide of Iron and an extra point of AC to the polearm fighter, as if they weren't already scary enough. Two AC with a rhythm blade, three with hafted defence, four with two weapon defence. *Winces*
Weapon loadout: Fullblade, staff of ruin, empty. Build: Swordmage
A fullblade wielding swordmage with a staff of ruin helping his implement powers off dual implement spellcasting and swordmage warding? Yeaaaah.
Saving the best for last:
Weapon loadout: Paired shortswords (lower arms), heavy shield, free (upper) arm for grappling or throwing javelins/handaxes. Build: Tempest Fighter.
It's a Tempest Fighter who's also carrying a heavy shield for defence - and can use Brawler powers. Here's a good idea...
That's both off the top of my head and before we start talking about hybrids.
Now a Thri-Keen paragon path to use all four weapons might be a good idea. I'm not sure after looking at my above builds.