hey goto this website
AWG to Metric Conversion Chart
look at table and tell me which one is your cable
your 4ga wire is it 4/0 awg or 4awg as they very different
I use 2 awg which is 33.5 mm2 and takes a 35 sized lug
anything smaller diameter cable can cause issues when battery is in the boot
I use this cable for everything relating to battery cables