My advice is since there is a contract in place, you should follow the letter of it. You shouldn't try to re-write it in "moralistic terms." Its' a slippery slope once you start going down that, who "deserves" what. Did she "screw" you by under-performing more than you "screw" her by firing her for under-performance? There are no answers.
So: Didn't work out, we had language in the contract that specifies what happens when it doesn't work out and there it is.
After three weeks, you'll be glad that you don't have 4% of "legacy equity" on your ownership table. And in 2 years, when your new, brilliant, terribly hard working engineer gets options for 0.5% you'll really be glad!