Nodal analysis of a basic DC circuit
You can't do a straightforward nodal analysis in this case because you can't directly write a term for the current through the voltage source. Instead, you need to create a supernode that encloses the voltage source. Write one equation that is the sum of the currents for the supernode (the currents through R1 through R4). As mentioned before, you need to mind the directions of those currents.

You need one more equation. That equation comes by inspection and simply specifies the voltage difference between the nodes at either end of the voltage source. Really, the first thing you should do is number all of the nodes in the circuit.