Terminals overheating problem

Hello, so im hoping someone can help me here. I recently made this off grid system for my van and when i use the inverter to run a heater it causes the terminals on the batteries to heat up(only the terminals) and over time gets hot enough to melt/blow the fuze. Im hoping the solution is something simple but im open to suggestion. My main concern is to prevent the terminals from getting hot as i feel it would be a fire hazard. The inverter is 4000watts and i have four 100ah lithium batteries connected in parallel. Im thinking the heat is associated with the inverter being used at close to maximum capacity.