So to get to weapons 4-5 you need a sword the size of a person which also happens to be sharp. I would max out melee base damage at 5 because anything above a car is silly even for someone with mythic strength.
I'd say that's reasonable unless you're talking about things that are substantially larger than human sized. For creatures with Hulking Size (which goes up to house size) swinging equally huge weapons I'd go up as far as seems appropriate. If a giant smacks you with his 15' battle axe I'd say Weapon:6 is entirely appropriate.
So long as the material it's constructed from is sufficient to maintain its form under the stress of such an impact (iron and steel aren't going to do the job).
If I really wanted to just add two to the weapons rating of a sword I would make it an item of power and give it a custom power like [-1] Monofilament Blade: Add +2 to the weapons rating and ignore up to +2 armour.