Compressors are generally used in two places:
1) For each vocal/instrument channel usually via the insert, to reduce the dynamic range on those performers who have a tendency to 'yell' and then get very quiet...
2) On the main outputs of the mixer before being sent to the amplifiers, to protect the amplifiers from excessive levels being sent by the mixer.
It's also important to remember that the 4 channel compressors don't normally have the same features as single or dual channel compressors. These may be features that you need to improve. On the other hand there may be things that don't need these features in which case the 4 channel compressor is ok. You may have a mixture of 2 and 4 channel ones. Remember, don't compress a channel because you have a spare compressor channel.
Hope this helps.