I assume by half as tall you mean it's size is scaled down by a factor of 2 ...... (otherwise it's weight would just be half of 400 lb).
Size scaled down by factor of 2 therefore volume scaled down by factor of 2^3 = 8 therefore weight is 400/8 = 50 lb.
The weight is 50 lb.
Aside to physics trained folk like topsquark: yeah yeah I know that's the mass not the weight, but when you look at the unit given for the answer to be in, hmpppph! ........ Another bug bear of mine is given at the end of this post.
If size is scale down by factor of 2, then area of feet is scaled down by a factor of 2^2 = 4. So area of feet is 1/4 ft squared. 1 ft squared = 144 inches squared therefore 1/4 ft squared = 144/4 = 36 inches squared.
So weight is 50 lb and area of feet is 36 sq in.
Now sub into your formula for pressure .....
Oh yeah, my other bug bear ...... maths books that give temperature in units of "degrees Kelvin" or . It's just plain Kelvin, fer cryin' out loud! All comment welcome (to get things totally off topic )