NOVELTY - Separation of a 4 carbon (C) hydrocarbon mixture (I) involves extractive distillation with N-methyl-2-pyrrolidinone (NMP) into 1,3-butadiene, butene and butane fractions and fraction(s) containing other 4 C hydrocarbons (II). The stages used comprise; (1) complete absorption of 1,3-butadiene and (II) from gaseous (I) with NMP and separating the solution (III) from a gas stream (IV) of unabsorbed butenes and butanes; (2) desorption of butadiene from (III) and separation from the liquid phase (V) containing most of (II); (3) desorption of (II) in one or more fractions from (V); (4) complete absorption of butenes in (IV) in NMP from stage (3) and separating the unabsorbed butanes; (5) desorbing the butenes from the solution; and (6) recycling the NMP free from stage (5) to one of the extraction zones. Each desorption zone operates at a lower pressure and/or higher temperature than the immediately preceding absorption or desorption zone.    USE - The process is useful for separating mixture (I) into a 95-99 wt.% or over 99 wt.% pure 1,3-butadiene fraction, together with a butene fraction containing at most 5 wt.% butanes and a butane fraction containing at most 15 wt.% butenes.    ADVANTAGE - The process is very efficient and economical. In particular, the amount of energy needed and capital costs are very low.    DETAILED DESCRIPTION - Separation of a 4 carbon (C) hydrocarbon mixture (I) involves extractive distillation with N-methyl-2-pyrrolidinone (NMP) into not less than 4 fractions comprising (a) a 1,3-butadiene fraction, (b) a butene fraction, (c) a butane fraction and (d) fraction(s) containing other 4 C hydrocarbons (II). The stages used comprise; (1) complete absorption of 1,3-butadiene and (II) from gaseous (I) with NMP and separating the solution (III) from a gas stream (IV) of unabsorbed butenes and butanes; (2) desorption of butadiene from (III) and separation from the liquid phase (V) containing most of (II); (3) desorption of (II) in one or more fractions from (V) and separation from NMP free from 4 C hydrocarbons; (4) complete absorption of butenes in (IV) in NMP from stage (3) and separating the unabsorbed butanes and solution (VI); (5) desorbing the butenes from (VI) and separation from NMP free from 4 C hydrocarbons; and (6) recycling the NMP from stage (5) to one of the extraction zones. Each desorption zone operates at a lower pressure and/or higher temperature than the immediately preceding absorption or desorption zone.    DESCRIPTION OF DRAWING(S) - The drawing shows the process schematically.    Stage (1) extraction zone (EI)    Stage (4) extraction zone (EII)    Stage (2) desorption zone (DI)    Stage (3) desorption zone (DII)    Stage (5) desorption zone (DIII)    Extraction solution (III) (Ead)    Extraction solution (VI) (Eb)    Solution (V) (Ed)    1,3-Butadiene fraction (Fa)    Butene fraction (Fb)    Butane fraction (Fc)    Desorbed fraction of other 4 C hydrocarbons (II) (Fd)    Gas stream (IV) (Gbc)