wanna know where to connect the line output signal from the HU to the line in of the amp, is a 4channel but the leads from HU are 2 (red & white subwoofer signal)see pic
While the above-mentioned method is possible it is generally inadvisable unless you are very good with a DMM and are fairly certain you can gain-match those bridged channels to give the exact same signal to each voice coil. Otherwise you'll risk sending unequal inputs to the different coils which will eventually lead to VC failure.
Why are you using a 4ch amp to power a single sub?
I had 2 subs but i was tryin to get to more power so decide to hook up one, but it`s true i hear the sub is playing like 2 signals, but still if bridge 2 subs to the 4 ch, where i connect the 2 rca leads from the HU? becuase ch1-ch2 will be for 1 sub ch3-ch4 for sub 2, so igot to connect let's say red line to ch1 and white to ch3?