40 (forty) is the natural number following 39 and preceding 41.

Despite being related to the word “four” (4), the modern spelling of 40 is “forty”. The archaic form “fourty” is now considered a misspelling. The modern spelling possibly reflects a pronunciation change due to the horse–hoarse merger.
In other words I’ll be celebrating my 40th birthday by playing music with three great bands and some special guests.