Classical Guitar - Britten Nocturnal - to 4 string Fretless Bass, How if it is ever played ?

Mustafa good question, and good choice of music - I've been stealing licks from Nocturnal for years. I've never heard it played on bass; in its entirety, it might - might - work on a 6-string bass, but otherwise all I've ever done with it is improvise off of fragments (in free-jazz contexts, or when noodling), or insert fragments into actual tunes - that doozy of a 10-note ascending line in mm 8 of Part I has been known to find its way into Pinocchio, for example, over an F13 #11.
By the way another piece you could try adapting is El Polifemo de Oro, by Smith-Brindle; a little more workable on 4-string, and those tone rows are money in the bank! And there are lots of others: Cavatina (Tansman), Drei Tentos (Henze), etc. I've thieved from them all, including the one below! Good luck.

Frank Martin - Quatre Pieces Breves Manuscript

 

I agree with vilshofen. I think a 6er would be more appropriate for the piece.