Another obvious but totally missed by me solution@t6blo or use a butt splice crimp to extend; 4awg = 25mm2
Butt Splice Connector - 25mm² Cable
Tinned copper butt splice connector used for joining higher current-carrying battery/power cables. Suitable for crimping 25mm² cable.www.12voltplanet.co.uk
Cover the join well with a couple of layers of adhesive lined heatshrink.