ContourPlot3D[4x^2+y^2==z,{x,-2,2},{y,-2,2},{z,0,5}, MeshFunctions->{#3&}]

ContourPlot[4x^2+y^2,{x,-2,2},{y,-2,2}]

ContourPlot3D[(E^(-x^2-y^2))==z,{x,-5,5},{y,-5,5},{z,0,1}, MeshFunctions->{#3&}]

ContourPlot[(E^(-x^2-y^2)),{x,-2,2},{y,-2,2}]

Plot3D[-x*y*(E^(-x^2-y^2)),{x,-2,2},{y,-2,2}]

ContourPlot3D[-x*y*(E^(-x^2-y^2))==z,{x,-2,2},{y,-2,2},{z,-0.5,0.5}, MeshFunctions->{#3&}]

ContourPlot3D[-x*y*(E^(-x^2-y^2))==z,{x,-5,5},{y,-5,5},{z,-0.5,0.5}]