Persepective mapping

Imagine a camera pointing down-ish on a rectangle laid out on the floor
Taking a pic - 4 corner x/y pixels values are
top left 73,100
top right 280,96
bottom right 300,230
bottom left 50,224
with a point p at px,py
i.e
    tl               tr

          p

  bl                    br

now I want to take point p and work out whats its position within the rectangle if the image was  transformed into a birds eye view

eg
tl                      tr


      p


bl                      br
(assume image is now width 360, height 240)
The question I need answering is how do I determine the new x/y co-ords of p within new frame 240x320 frame of ref