Erin's a funny one.
I agree with a pp that it "barely sounds like a name", and I also associate it with heron.
However, I do actually quite like it, and out of your four it would be my preferred name.
Molly; agree with a pp, very dated and infantile.
Grace; so horribly overused. People seem to think it's incredibly classy for some reason, but it just isn't. I also just don't like the sound, I don't like the hard G.
Isla; have always loved it but the popularity would sadly put me off a lot. It's sudden rise in popularity will mean it dates very quickly (it already sounds dated to me).