the kg4s were from the first run of kg speakers by Klipsch...they were made a long time and have a fierce following...I think the KG 4.2s are more efficient and might even go lower but I bet most would get the kg4s...
http://www.klipsch.com/na-en/products/kg-4-specifications/
http://www.klipsch.com/na-en/products/kg-4-2-specifications/
the kg4s have a rear firing passive radiator so placement might be more difficult...the KG 4.2 has a passive in the front so placement is easier...the 4.2s are newer...good luck with your choice...
Bill