Do it by hand / ear, but don't just pick the loudest.
Import the 4 tracks into Audacity.
If they should be 2 pairs of stereo tracks, then join the pairs so that you have two stereo tracks (see: http://manual.audacityteam.org/man/spli ... racks.html
Use the track Mute or Solo buttons to listen to one track at a time (see: http://manual.audacityteam.org/man/audi ... .html#mute
Use the Track Gain sliders to make both tracks sound about the same loudness, but not so loud that they distort (see: http://manual.audacityteam.org/man/audi ... .html#gain
Listen through good headphones or good speakers and choose which sounds the best
Delete the track that sounds inferior (click the [X] in the top left corner of the track)
With the good track selected, use "Mix and Render" from the "Tracks" menu.
Then use the Normalize effect and normalize to -1 dB (see: http://manual.audacityteam.org/man/normalize.html
Then Export as a WAV file and name it appropriately (example "my-recording-best.wav"). Don't overwrite your original files, keep them safe just in case you need them again.