I got a 02 f150 4.6 and I'm getting all 4 cats cut and and replacing them with 2 magnaflow cats PN: 94306 I was wondering if and how much of a fuel economy increase I could see

 

There is already a thread below called 'hi flow cats'.
Look at it for some other info..
Basicly, replacing cats, you need to really know if the replacement cats are higher flowing with some hard numbers, not just words.
You already have cat flow capacity greater than the exhaust gas flow the motor produces by factory design.
Unless your cats are beginning to get plugged or broken up internally you won't see a 'worthwhile' change in fuel mileage.
Read the last statement again.
Often times replaceing the stock type cats "and" Ox sensors will improve fuel mileage just because of age, not from a change (to) high capacity flow cats.
From experience I have just had replacing the right side cat assembly that got burned and melted from a massive right bank failure of the PCM, the fuel mileage has returned to a nice 15.5 to 16.5 over the road mileage and the truck feels much smoother and the trans even shifts a bit different from the extra power (throttle position) thats back as it was before the event.
Read and think about it and make your decision.
Good luck.

 

Ok thanks. My cats smell like rotten eggs sometimes and my exhaust got so hot it turned my muffler that's a few weeks old black and the low end power seems to have gone down so that's why I'm replacing the cats. I just wondered if there is a advantage over a stock good working system.

 

There may be a marginal improvement from a numbers standpoint, but it's nothing you'd notice going from a stock *good working* system. I changed mine out just because they sounded better, otherwise my old cats were fine.