How to bridge a 4 channel amp.

Use the left positive and right negative. ..

Its a 4 channel. Channel 1 + and channel 4 - ?

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Channel 1 + and 2 -

^^^This.

Ok thanks. Can I use 3 & 4 as well

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Using a 4-channel amp is not optimal to power a single subwoofer. I'd trade it for a mono block or a capable 2-channel amp. Actually, in your case, you can't. If you tried to power each voice coil with a bridged set of channels, they would have to be 2ohm stable in bridged mode. Not a good idea IMO.

for most 4ch amps yes you can bridge 1+2 and you can bridge 3+4, making most of those amps "2/3/4" channel amps. You'll need to put the coils of your sub in series for 4 ohms. You can use the 2 other channels for front speakers or something.

As for how to accomplish it, that can vary per amp. Some will connect the negatives, flip a switch, and use the two positives for the speaker output. Some will connect L ch - to R ch + and use the other two for speakers. Some will connect L + to R - and use the other 2 for speakers. etc. Best to look in the amp's manual if it's not silk screened on the amp how to do it.