hey just wonted to no if i use xr6 computer with a 4ltr with cold air and big cam would it make a diffrance to the power

In my opinion, no.

ok. i wont more power whats the cheap way

here you go.. read through this 12 pages of ford six mods...

http://www.fordmods.com/forums/viewtopic.php?t=66

"mods" and "cheap" usualy dont go together..

although "cheap" and "nasty" do.....

how much would you pay for xr6 head

The xr6 computer is tuned for the high rate fuel pressure regulator.

So in theory it should run leaner with a stock 270kpa reg

In my experience, the XR6 computer will learn around the lessened fuel pressure.
Oxy sensor feedback in closed loop will inform the ECU it is running lean, and it will make changes across the board (learning), to richen it back up.
I have no idea to what level it can learn, for instance how lean/rich before it runs out of adjustment.
If you whack an XR6 ECU in your stock/mildly modifed 4L, you will notice one of two things.
It will feel quicker for a little while till it learns around the lean condition, and two, if its an auto your shiftpoints will be f**k and it will be shifting too early.
I theorise it could also handle slight increases in injector size this way, and is also the reason why, for a MAP based ECU, it can handle some fairly serious mods before it calls it quits.