You'll probably see some benefit, but I doubt it would be huge.
Oh, and obsidian: quadfire definitely does have a use outside of benching. Since I added a second 4870x2 to my setup, Crysis runs smoother at 2xAA than it used to at Very High with no AA, and almost all other games run at 8xAA fully cranked (all at 1920x1200). The higher the resolution, the more worthwhile high end CF/SLI is. Still, if you already have a trifire setup, there's questionable benefit to adding the 4th GPU. It'd probably be better to wait a bit, and maybe upgrade to a 5870x2 in a few months.