By contrast, try to throw a sword of rat-farting into a 4E treasure parcel, and the system simply won't compute ("##error - hand slot entry: useless")
How? Presumably, like every other magic weapon in the system, it will have an enhancement bonus of ceil(level/5). And presumably the power you put on it will be useful.
Let's compare the systems (it's been a while since I've looked at my 3e books so I might be getting something wrong there - correct me if I'm wrong about anything)
In 3e, you put whatever powers you want on a magic weapon. You calculate the cost as (2000 * (n+k)^2) + c, where n = enhancement bonus of item, and k = sum of "bonus equivalents" of all the powers you put on, and c = "extra cost" for those enchantments that just have extra costs, not bonus equivalents.
In 4e, you put a power on an item, and calculate the level of the item as (5n-4)+k, where n = enhancement bonus of item, and k = "extra levels" the power is worth, which goes from 0 (basic magic weapon) to 4. If you want to extend this system to combine multiple powers on an item, it's pretty simple, just add the values of k together. For example, if you want a weapon that is +2 has the powers of both a duelist's weapon and a frost weapon, you could make that, and it would be level 10 (k=2 for duelist's weapon, k=2 for flaming weapon, just add them together). Or if you want to make your own weapon power you can just decide how much "k" should be by comparing it to existing powers.
Like I said, I think that if we understood exactly what kind of item you want to make, we could explain how to make it in the existing system. I don't know what a "sword of rat-farting" is supposed to do, so I can't help you there - but if you told me what power you wanted it to have I could explain how you could make it.