The layout "dot" provided the interface to graphviz seems to do what you are looking for. While it works for this simple graph you've provided, you may need to tweak the output for larger more complicated graphs.
import networkx as nx
G = nx.DiGraph()
G.add_node(1,level=1)
G.add_node(2,level=2)
G.add_node(3,level=2)
G.add_node(4,level=3)
G.add_edge(1,2)
G.add_edge(1,3)
G.add_edge(2,4)
import pylab as plt
nx.draw_graphviz(G, node_size=1600, cmap=plt.cm.Blues,
node_color=range(len(G)),
prog='dot')
plt.show()