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You might be able to get implicit_plot3d to do it. Maybe something like the following would work for you.

var('x,y,z')
f(x,y,z)=x^2+y^2+z^2/4
p=Graphics()
for k in [1,2,3,4]:
    p+=implicit_plot3d(f(x,y,z)==k,(x,-2,2),(y,-2,2),(z,-5,5),opacity=0.4)
p