How can I setup a 4.1 system?

[PhilipKapadia]

Hi all,

How might one set up a 4.1 audio monitoring system via an audio mixer? 4.1 I understand is L, R, then Back L, Back R, then the subwoofer.

Is it okay to duplicate the forward and backward speaker channels, or do they need to be separated into separate channels like in a 5.1 system?

I have an audio demuxer coming out of a video SDI cable, then I want to patch the 4 channels out of the demuxer to a mixer that can output to all of my 4 speakers + sub. What sort of mixer do I need to output 4.1? All the mixers I see online only have stereo outputs.

Thanks in advance, apologies for my basic understanding.