vplot = VectorPlot[{x - 4*y, x-y},{x,-3,3},{y,-3,3},VectorScale->{0.04,0.04,None},Axes->True]

splot = StreamPlot[{x - 4*y, x-y},{x,-3,3},{y,-3,3},Axes->True]

nulplot = Plot[{x/4, x}, {x,-4,4}, PlotStyle->{Red, Black}]

Show[vplot, nulplot]