4500rpm does seem a little high, When I drive around town mine shifts at about 3000rpm is im granny driving it, if I drive normal it shifts around 3500rpm-4000rpm. If you get on it a littel bit it wont shift till over 5000rpm. If you floor it, it will shift at 6500rpm-7000rpm. All this is with OD on. If its off you rev higher before you shift becasue it basicly turns off 4th gear.
I would say to change your tansmissions speed sensor.
Part Needed: 1S7Z-7M101-KA (Turbine Speed Sensor) - $15 at FordPartsOnline.com
Its cheap and easy, and most problems with our ATX start with it. All you need to do to replace it is pull off the drivers side wheel and you can see it right there on the side of the trans. Its plastic and has one bolt holding it on. Just swap it for a new one.
They talk about it here
http://www.newcougar.org/forums/how-...ang-shift.html