<< Graphics`PlotField`;

PlotVectorField[{1, (1/1000)*y*(4000 - y)}, {x, 0, 100}, {y, 
          0, 4500}, ScaleFunction -> (1 &), 
        PlotPoints -> {25, 25}, AspectRatio -> 1/GoldenRatio, 
        HeadWidth -> 0, Axes -> True];

?ScaleFunction

?? PlotPoints

?AspectRatio

?GoldenRatio

?HeadWidth

?Axes