I have 99 with the stock air intake. Been thinking about a cold air for a while now but they are rediculusly expensive. Anyway, a buddy of mine gave me a cylinder-type K&N filter which is 11 inches long and 6 inches in diameter. Plenty big for a filter. I was going to fabricate my own box and tube. The question is, the filter has a 4 inch opening and my current box has a 4-1/4 inch opening, will it restrict me if I use a reducer from 4-1/4 to 4inch or will it make any difference?

 

without serious power (500+) it won't make much difference. K&Ns will get oil on your turbo impeller if over-oiled. A lot of the air filters can use the stock intake tube and slip the tube over the outside diameter of the filter. a small piece of 4" exhaust tubing (like 1" long) inside the filter neck can add support.
Also, maybe do a search for BHAF (big honkin' air filter) for a dry alternative that flows and filters well.