Damage should be increased drastically, as that happens with just about any weapon going from 3* to 4*. At least 3 bars of damage more than the 3* would be okay.
I would say keeping the curse level being moderate opposed to strong. At moderate we have a less powerful curse and whatnot, but it can be resisted. Keeping the curse on moderate also balances the weapon a bit more in regards to the PvP scene without really harming the PvE style of it.

Perhaps at 3* it has a slight chance of minor curse, and at 4* a good chance at minor curse? If not that, then 4* would be a slight chance at moderate curse.

That seems reasonable in terms of damage now! Keeping moderate curse is good.
If this sort of thing is added I'd like to see what is in stone as of the last post on a testing server to see how well it fairs. Good chance of moderate curse seems alright at face value, but I'd need to see it in action to really get any further thoughts on it.

Not really a "mainstream" thing so much as it is a balance thing. If the weapon doesn't deal damage then it's essentially a curse mist bomb, except it's not. Having it compete with other damage bombs, especially since the only really good shadow bomb is a DLC one, is a good idea. Curse is a status we need to see more of, but a fair source of damage here would be good.

Again, I'd like to see such a thing in action before I go about saying how "mainstream" it is. If the impulses are slow and predictable, it might be interesting , and if they're fast and whatnot they'll be a different sort of bomb altogether.

A bomb being "mainstream" isn't really a bad thing. In fact, it's a good idea to have bombs that look and act in a neat way to encourage new bombers. :)