4" is fine enough.
This question is not n00bish, I had a tough time understanding Z at first too, but it isn't too bad at all. I did it myself and I'm a car idiot.
First thing to do is get the necissary parts for the procedure. I don't know what year you have, but you need the proper pieces to plug up the holes left by taking out the silencers. Then you need the deckplate and screws, some sort of sealant...etc
Once you have everything you need, you need to take out the intake. Unscrew the two bolts holding down the airbox and loosen any hose clamps on the intake (they are metal rings with a phillips head screw that tightens it). The airbox is loosely connected to both the fenderwell (should come out no problem) and the mass airflow sensor (MAF), which is a little cylender connecting the airbox and the tube the silencers are connected to. be careful with the MAF! make sure to unplug it!
Once the airbox is out, put it aside with the MAF and undo the hose clamp holding the tube with the silencers together with the metal intake. take off the tube and loosen the hose clamps that hold the silencers and take them all off. put in the PVC caps to replace them and retighten the hose clamps and make sure the caps have no room to wiggle. If there is a bit of movement, just do what I did (I dont have a pic but I'll post one in the morning, I'm too tired to go outside now). then back to the airbox. take the filter out. Then use a dremel or any other cutting tool to cut the hole in the front of the airbox (use the deckplate as a template to draw a circle on the box, then use that circle to cut it out). file it smooth. place the plate in and use a pen to dot where the screws will go and drill some small pilot holes. put a ring of sealant where the plate will rest and pop that sucker on, then screw it down. the next important step is to vaccum the crapola out of the inside of the box so that no rogue shards of plastic find their way into the combustion chamber. this is the step that took the longest for me.
clamp all the crap back together and off you go. make sure to reset the EFI.
I don't know if that is much easier to understand, maybe explain one certain part that you cant figure out and I can be more detailed...