It is definitely venue specific. If it is a large outdoor festival type venue, you can get away with the 20' spacing. If it's small you can do what Craig did and stay close. Generally, 3-7 feet spread will get you nice results. You have to watch out about spacing it too far or you will get the hole in the middle effect: Only left and right characteristics of the recording, no middle.
Other than that, omni records can turn out great or turn out poor. They are not as forgiving as normal directional mics capsules.
One interesting omni note: the time difference of the arriving signal can make a bigger difference than you would think, also compared to typical directional capsules. One venue (Mishawaka) has a stage slightly angled. The left mic was recieving the signal a few miliseconds earlier than the right. You wouldn't notice this unless you can see the waveforms on a computer software program. So, by lining up the 2 channels, the recording cleared up and a lot more definition was produced.
It's amazing what the human brain can process and understand!!
Rob-you should be ok with your 4' bar.
Good luck
Phil