To make a Mod 4 out of a Mod 0 (the basic gun) you would need to add:
Reduced Power Hammer Spring
ROBAR NP3 finish package
Full Langdon Trigger Job
Just adding a hammer spring is easy. The Wilson 12# is great, but Langdon's new 11# might be worth a try.
Or, you could easily order one of Langdon's "trigger job in a bag" and get much of the benefit of the full langdon trigger job, although it's not quite the same as having him work it over fully. Note it comes with a hammer spring.
And of course the NP3 package is it's own animal to add.