Answer
Change in internal energy of the gas when it is taken along the straight path from $a$ to $c$ is -4$J$.
Work Step by Step
Internal energy is an state function i.e it depends on state of gas, no matter how the gas achieved that state.
So, (Change in internal energy in the process $ab$) + (change in internal energy in the process $bc$) = change in internal energy in the process $a$ to $c$ directly
Process $ab$ is isothermal so, change in internal energy is $0J$
Process $bc$ is adiabatic so, Change in internal energy is $(-W)$ where $W$ is work done.
So, Change in internal energy when it is taken along the straight path $ac$ = $0 + (-4)=(-4)J$
