[co9ol]

if you have 4 sources that means (assuming they are stereo) that you have 8 mono channels

5.1 is actually 6 channels
L______C______R

______LFE______

LS____________RS

L=left (mono)
C=Center (mono)
R=Right (mono)
LS=Left Side aka the left side that's behind your head (mono)
RS=Right Side aka the right side that's behind your head (mono)
LFE=Low Frequency Emiter
the LFE is the .1 of a channel because it is only used for sounds below 120Hz

all of those mono channels make up 5.1 surround sound (the same way that 2 mono channels make up a stereo one.

How you mix this show all depends on how those sources sound. If some of them don't sound that great then pick the best two (the closer sounding one for the front channels and the farther sounding one for the rear channels) or your could use all 4.

A few things to remember:
sound travels front to back
LFE is for 120hz and below only!
and you do not have to use the center channel (unless your mixing an album, it's really more for TV and movies)