Grab the popcorn! pull up a comfy chair this should be good.
Rats, beat me too it...
Anyway, Hiya Mud16,
Don't bother at the moment.
Hydrogen does work great as a hydrocarbon catalyst or as fuel in itself, indeed in the US you could drive one end to the other on Hydrogen fuel with delivery stations along the way and the WA Gov were running a 100% Hydrogen powered bus trial here in Perth for a few years but it was uneconomical in scale IE not enough users to justify the Hydrogen price.
For the 4.8 you will need too much engine bay space, too big a fuel cell bank and most of all too much electrical power (50 Amps or more) to run the Fuel Cells for onboard generation of sufficient HHO for that size engine to make a difference worth doing.
...and the other posts are correct in that most of the Ebay or backyard solutions are, unfortunately, Snake Oil. The ones that are somewhat efficient and safe are BIG $ in the scheme of things and don't run on just water, they use a chemical mix
There is a bottom line however and that is this... A lot of us spend waaay too much dollars and time on stuff that is either fun, a hobby or just out of plain curiousity so if experimenting with HHO is of an interest to you then I say go right ahead. You can always pull it out if the novelty wears off... just don't do it if you are expecting to recoup the $ and gain astronomical power increases etc