Complex sentence: Another important festival in Iida is Oneri Matsuri which is held every seven years (the last being 2004), a massive four-day event, most famous for the shi-shi lion dance.
Split into simpler sentences: Another important festival in Iida is Onaeri Matsuri which is held every seven years (the last being 2004). It is a massive four day festival, most famous for the shi-shi lion dance.