A while back I modified some hubs and rims for "Big" Guy (Teardrops for Vets). Toyota uses a 5 X 4.5 on some of their vehicles. He wanted to use Toyota alloys to match his car on his trailer. We drilled out the holes for the studs to 14mm (as I remember) and pressed in metric studs. We also had to bore out the hub of the rim to fit over the hub of his new axle so you may also have to bore out your rims to fit over a trailer hub. In addition, the trailer hubs stick out more so you probably won't be able to put the original hub cap back on.
However, if you want to procede, check with Century Wheel and Rim in the LA area and see if you can order an axle with blank hubs or brake drums.
Here is their web page:
http://www.qtrailer.com/locations.php?comp=CWR
click on the "undefined" dot in the middle of the Pacific Ocean on the map!
If you can find blank drums or hubs they can be drilled to any pattern you wish. If you can't find blank drums or hubs it would be possible (IMHO) to drill 5 new holes inbetween the original holes. If you need 4 holes that would present a problem unless you can find an axle with the capacity you want with a 4 bolt pattern.
If you want to procede I could do the machining on my mill and it would take a morning or so. I'm in Simi Valley about a hour North of you. PM if you have more questions.
Cheers,
Gus