I think your 4-channel amp is rated max, so in continuous power output, at best it'll be just 4 x 75w RMS.
Use this to power your front speakers full active and bridge your 2 channel amp to whack the sub.
At the moment, in my Kancil, I am using only one single 4 channel amp, 4 x 75w RMS,--2 channels use to power the front speakers through passives and the remaining 2 channels use to power 2 units of 8" subs, free air, mounted on the rear parcel shelf.