Complex sentence: The source of inspiration for "The Water Goblin" was a poem found in a collection published by Karel Jaromír Erben under the title "Kytice"; while Dvořák composed six symphonic poems, four of these were inspired by works of poetry found in that collection.
Split into simpler sentences: The source of inspiration for "The Water Goblin" was a poem found in a collection published by Karel Jaromír Erben under the title "Kytice". Four of the six symphonic poems that Dvořák composed were inspired by works of poetry found in that collection.