If you are in a situation where a four-channel version of an original two-channel live recording is required, you can create one using Altiverb!

Set up a stereo input, four-channel output Altiverb, and play back the material through it. Select a hall that seems to match the hall that the original recording was made in. The front outputs of Altiverb should be quite low so as not to interfere with the original master too much. The original master is fed to the front channels only.

Now start adding rear channel reverb to a point that you are barely noticing it, and then add another 3 dB. This is a good level to determine which hall to pick. If you now select a variety of hall and microphone configurations, you can easily decide if the reverb sticks to the original or not.

If you have a hall that matches, you will find that the four-channel version you are listening to is perfectly convincing without destroying the original master in the front channels. This works best on acoustic live ensemble or solo recordings, although studio performances can become convincing surround masters as well.