let's convert 400 newtons to Kg, which is 40.7886.
now we put it in a formula that includes gravity and acceleration
(40.7886 kg) ( 9.8 m/s2) + (40.7886 kg) (0)
now we come up with the answer of 399.72828 kg or 3920 Newtons
Not sure which unit you are looking for but they are both there.