Yep BAF is right
The lift kit lifts the body and or the frame. The tires lift the differential... So a 23" tire is 5 over stock. Half of that is on the bottom of the hub lifting the differential ...... you guessed it 2.5" of actual improved diff. clearance..... Don't let that fool you...Those tires require a 6" lift kit. That will raise the frame a total of 8.5". You prolly had about 7 to start with so now you have 15" of ground clearance except at the diff it is prolly half of that... A good driver could go about anywhere you might dare to go with that kinda clearance
To answer your second question about a 4" lift..... I think you can run 22" tires on a 4 inch lift... You can do the math
