3-way merge tools for Mac that show 4 panes

I'm a fan of the 4-pane approach to 3-way file merging. In particular, being able to see Branch A, Branch B, and the ancestor of A and B in 3 read-only panes, then a 4th read-write pane which shows the results of your merge.

However, I've been unable to find a 4-pane merge tool for Mac OSX that I can integrate with mercurial/git. Does such a beast exist?