What does the system shown in above figure represents?

The DC transmission systems are classified as:

DC two-wire

DC two-wire with mid-point earthed

DC three-wire

(i) DC two-wire:

• In the 2-wire DC the system, one is the outgoing wire called positive wire and the other is the return wire called negative wire as shown in Figure below. 
• The return wire is also connected to earthing.

(ii) DC two-wire with mid-point earthed:

• The two-wire DC system with mid-point earthed is shown in the figure below.
•  The maximum voltage between any conductor and earth is Vm and the maximum voltage between conductors is 2Vm.
• It increases the voltage level in comparison to two-wire systems.

(iii) DC three-wire:

In a 3-wire DC system, there are two outers and a middle wire which is connected to earth.

When the load is balanced, the current in the neutral wire is zero. 

DC 3- wire system is shown in figure below.

Conclusion:

From the above concept, it is clear that the DC four wires with line earthed are not considered for DC transmission.