I have not (actually, I typically use Wyeast, so I've never used WLP004) but I was just planning my weekend brew and was thinking about doing a side-by-side with my usual stout with 1084 (Irish Ale) and 1968 (London ESB) to see how they come out.
I don't see why you couldn't make a great mild with Irish ale yeast. It may be fruitier? But lots of the English ale yeast are fruity, too.