We report the detection of the linear radical HC4N in the C-rich envelope of IRC+10216. After HCCN, HC4N is the second member of the allenic chain family HC_(2n)N observed in space. The column density of HC4N is found to be 1.5 10**12 cm**(-2). The abundance ratio HC2N/HC4N is 9, a factor of two larger than the decrement observed for the cyanopolyynes HC$_(2n+1)N/HC_(2n+3)N. Linear HC_4N has a 3-Sigma electronic ground state and is one of the 3 low-energy isomeric forms of this molecule. We have searched for the bent and ringed HC4N isomers, but could only derive an upper limit to their column densities of about 3 10**(12) cm**(-2).nullPeer reviewed