The work was dedicated to Count Franz von Oppersdorff, a relative of Beethoven's patron, Prince Lichnowsky. The Count met Beethoven when he traveled to Lichnowsky's summer home where Beethoven was staying. Von Oppersdorff listened to Beethoven's Symphony No. 2 in D Major, and liked it so much that he offered a great amount of money for Beethoven to compose a new symphony for him. Beethoven undertook the new work during the summer of 1806 and completed it in roughly a month, while working on the Fourth Piano Concerto and revising his opera Fidelio, then still known as Leonore. The dedication was made to "the Silesian nobleman Count Franz von Oppersdorf". Hector Berlioz was so enamoured of the symphony's 2nd movement that he claimed it was the work of the Archangel Michael, and not that of a human.
In general the symphony is sunny and cheerful, with light instrumentation in a manner that recalls the symphonies of Joseph Haydn, with whom Beethoven had studied a decade before. The Fourth Symphony contrasts with the swooping changes of Beethoven's composition style in the previous Third Symphony, and is often overshadowed by both its predecessor and following work, the celebrated Fifth Symphony which Beethoven had set aside to complete the Fourth. Despite being written in a style more akin to that of Beethoven's first two symphonies, the Fourth contains many aspects that show his growing strength as a composer, most notably the B-flat minor Adagio introduction to the first movement, which Leonard Bernstein described as a "mysterious introduction which hovers around minor modes, tip-toeing its tenuous weight through ambiguous unrelated keys [with reluctance] to settle down into its final B-flat major."
Source: Wikipedia