A process for controlled manufacturing of olefin trimers and olefin tetramers comprising: a. feeding at least one olefin monomer and at least one oxygen-containing moderator into at least one first reactor unit containing a catalyst; b. operating at least one of the first reactor unit(s) at a temperature selected from the range 30-140°C and at a pressure selected from the range 800-5000kPa for carrying out catalytic reactions between olefins inside the first reactor unit; c. withdrawing a first reactor outlet stream from at least one of the first reactor unit(s); d. distilling the first reactor outlet stream in a first distillation column to separate at least a first distillate comprising at least unreacted olefin monomers, and a first bottom product comprising at least dimers of the olefin monomers; e. feeding the first bottom product into at least one second reactor unit containing a catalyst; f. operating at least one of the second reactor unit(s) at a temperature selected from the range 30-140°C and at a pressure selected from the range 150-5000kPa for carrying out catalytic reactions between olefins inside the second reactor unit; g. withdrawing a second reactor outlet stream from at least one of the second reactor unit(s); and h. distilling the second reactor outlet stream in a second distillation column to separate a second distillate comprising at least olefin dimers, and a second bottom product comprising at least trimers and/or tetramers of the olefin monomers; and wherein the process further comprises controlling the composition of the second reactor outlet stream by: i. feeding olefin dimers into at least one of the first reactor unit(s) to increase the amount of olefin trimers in the second reactor outlet stream; and/or ii. feeding olefin dimers into at least one of the second reactor unit(s) to increase the amount of olefin tetramers in the second reactor outlet stream. a. feeding at least one olefin monomer and at least one oxygen-containing moderator into at least one first reactor unit containing a catalyst; b. operating at least one of the first reactor unit(s) at a temperature selected from the range 30-140°C and at a pressure selected from the range 800-5000kPa for carrying out catalytic reactions between olefins inside the first reactor unit; c. withdrawing a first reactor outlet stream from at least one of the first reactor unit(s); d. distilling the first reactor outlet stream in a first distillation column to separate at least a first distillate comprising at least unreacted olefin monomers, and a first bottom product comprising at least dimers of the olefin monomers; e. feeding the first bottom product into at least one second reactor unit containing a catalyst; f. operating at least one of the second reactor unit(s) at a temperature selected from the range 30-140°C and at a pressure selected from the range 150-5000kPa for carrying out catalytic reactions between olefins inside the second reactor unit; g. withdrawing a second reactor outlet stream from at least one of the second reactor unit(s); and h. distilling the second reactor outlet stream in a second distillation column to separate a second distillate comprising at least olefin dimers, and a second bottom product comprising at least trimers and/or tetramers of the olefin monomers; and wherein i. feeding olefin dimers into at least one of the first reactor unit(s) to increase the amount of olefin trimers in the second reactor outlet stream; and/or ii. feeding olefin dimers into at least one of the second reactor unit(s) to increase the amount of olefin tetramers in the second reactor outlet stream. <EOS>