RegionPlot3D[
y >= 0 &&
y <= 2 &&
x <= 4 &&
y <= Sqrt[x],
{x, 0, 4}, {y, 0, 4}, {z, 0, 4},
AxesLabel -> {x, y, z},
Mesh -> True,
PlotRange -> Full,
PlotPoints -> 100,
MaxRecursion -> 15,
WorkingPrecision -> 255]