From what I've read, the difference in meter/features on the om4 over the OM2-SP are significant enough for the extra money (not to mention they're both pretty cheap anyway!).
..yea, makes sense.
To be honest I'm not that familiar with the OM series, but I guess if a OM-4 isn't much more expensive I'd go for it as well to get some more features and to get a less old camera that's more likely to be in good shape.
From what I've read, the difference in meter/features on the om4 over the OM2-SP are significant enough for the extra money (not to mention they're both pretty cheap anyway!).