Kimber's premium aluminum triggers are set to 3.5-4 pounds.  What would it take to bring it down to a specific weight?

Are drop-in parts available, or do I have to send it to a gunsmith?

Not to mention that I've yet to see a "tuned Kimber trigger" at 3.5 to 4 lbs.  4.5, sure.  You need to send it to a smith, or you can try a trigger job yourself.  I wouldn't recommend the latter unless you have enough familiarity to detail strip the gun and realize the function of the various parts.

Kimber parts are MIM and reportedly don't hold well to grinding and polishing for trigger jobs.

You need top quality alloy tool steel parts for light trigger jobs just so that you and others around you stay safe.

I'm not saying it can't be done with the stock Kimber parts, but eventually the trigger pull will get heavy again.

Get good quality parts if you do decide to mess with the trigger pull.
Hammer
Sear
Sear spring
disconector
Pins
The stock trigger is OK.