You could do it in D&D via aid another; or, better still, in Iron Heroes via a combination of aid another and attack challenges. Thus, if four goblins attacked one fighter, they could take three aid anothers (+6) and a moderate attack challenge (-4) to make a single attack at +2 with a +3 to damage, or three aid anothers and a major attack challenge to make a single attack at par with a +6 to damage. You could just condense the whole process and tweak the math a bit, and do it like this:
Goblin
[rest of stat block]
Attacks: morningstar +2 melee (1d6) x4; or morningstar +4 melee (1d6+3)
Four goblins then effectively make one attack at roughly the same value as a bugbear. Seems fine to me. (In fact, it looks like I've got a new house rule!)