Small bolt vs. Big bolt

Yea, only one company makes a small BC kit for disc's up front. I don't beleive any one makes a kit for rear dics in 4"BC.
You get way more wheel choices with the 4.5 BC.
There is a company out theer called Coys (http://www.coyswheel.com./) that does make 4" BC wheels in 16 and 17 inch wheels, so it just depends on what you want and how big your budget is.