Klaestron IV was the fourth planet of the Klaestron planetary system. This was the homeworld of the Klaestrons, a warp capable humanoid species. (DS9: "Dax")
Klaestron IV had a capital city and an inn outside the capital where Curzon Dax and Enina Tandro spent some time. (DS9: "Dax")
In 2371, Julian Bashir visited Klaestron IV to learn a new treatment for burn victims. According to him, the technology had to be seen to be believed. (DS9: "Second Skin")
The image of the surface of Klaestron IV shown in "Dax", was one of many reuses of the matte painting for Angel I. However, the image of Klaestron IV at night had a new building, that wasn't present in the day shot of Angel I, and was so forgotten in the day shot of Klaestron IV.
According to Startrek.com, the pronunciation for Klaestron is "CLAY-stron". [1]
According to the Star Trek: Star Charts (pg. 47, "United Federation of Planets I"), the Klaestron system was located in non-aligned space between the Cardassian Union and the United Federation of Planets, in the Alpha Quadrant. This system was a single star system. Primary was a class K star with a magnitude of +5, which was the same brightness as Sol. Klaestron IV was listed as a member planet of the Federation in 2378.