4 pole to 3.5mm female
Its difficult to say for sure without seeing the circuit, but its likely that you shouldn't do that.
My reasoning for saying that is that since the manufacturer has specifically exposed &amp; labeled a + and - for each output, I think that each is being driven by a full-bridge/differential output amplifier.
This means that L- and R- are not simply GND/0V but are actively driven with a signal of the opposite polarity to L+ and R+.  

As a simplified example, this is what I think you have for each channel:







simulate this circuit &ndash; Schematic created using CircuitLab

However, my assumption could be incorrect...
If you take a multimeter and measure the resistance between your L- and R- (with the power turned off for at least a few minutes and nothing else connected) and find that they are connected to each other internally, then you probably don't have a bridge/differential amp and it is probably safe to connect L- and R- together.