A term many ska fans debate about. Some insist that Christian Ska is fourth wave, though musically it is still third wave. Other think fourth wave is heavier, though ska of that type is called Skacore. Most believe fourth wave is being lead by Thomas Kalnoky's Bandits of the Acoustic Revolution. It is a fuller band, similar to Big Band music, though with a more folk-y Eastern European sound. Similar to Gypsy music.
Fourth Wave includes a larger horn section and string section.
BotAR is fourth wave, the next ska wave.