Not sure where your getting it or which 4.5" RC kit you are getting so general list
extras, (the short list)and what I used(am at 6" lift now)
1. new brake hoses front and rear (92 Chevy 1/2 ton lines, rear 93 or so Dakota line)
2. might need a s.y.e (slip yoke eliminator)for the transfer case (I used tom woods)
3. if need above will need new rear drive shaft (i used tom woods)
4. will/may need adjustable control arms(atleast uppers) to adjust the caster angle when done
5. degree shims to correct pinion angle in the back
6. new e-brake cables (i use 2 YJ passenger side cables)
7. longer or adjustable sway bar links
8. steering stabilizer, maybe, I now alot of people that run with out them and have no issue on/off road (i use a rancho one from work)
If you are getting the add a leaf save your money a bit longer and get the full leaf pack. The add a leafs sag really quickly whether it is the short add a leaf or the long add a leaf.
Thats all I can think of at the moment.
This site is filled with these same questions "what do I need at such and such lift height". If you do a search you will find the same info as I typed above and more. I tell you this as we have alot of people that will just say "SEARCH it has been covered" so do not be put of by them. This is a great group of individuals with the same hobby, wheeling and having fun. Enjoy and good luck with the lift.
P.S.----Do not forget to soak all the bolts that will need to come off in PB Blaster or similar penetrant fluid days before the install. It will make it much easier.