Almost anything is possible with enough time and money. However 400 HP N/A does not seem practical considering that in spite of its 4V heads & larger displacement, the 5.0 Cammer puts out 420HP, is not 50 state emission legal and does not meet Ford's durability requirements for production cars.
A Ford engineer I spoke with told me that so far they have not been able to make 400HP with the 4.6L, 3V motor, at least not in street legal form.
Now if you have money to burn you could put in 92mm bore liners, which would bump you up to 4.8L, then put in a stroker crank and rods to get you up to 5.0, get new pistons that would bump the CR up to 10.5:1 (91 octane required), put in agressive cams, long tube headers and a cold air induction system, you're able to re-program the PCM and you don't have to worry about passing emissions, 400 HP may be in sight.