Fiber pull-out and fiber fracture are the two dominant failure mechanisms of softwood paper. For the first time, 4D synchrotron X-ray tomographic imaging (three spatial directions plus time) was performed to observe these mechanism in-situ during tensile deformation of softwood paper handsheets. The experiments were conducted on three handsheets, produced from pulp that was low consistency refined at 0 k Wh/t and 100 kWh/t and wither air-dried in restraint or freeze-dried. The fibre deformation was found to be highly complex; initially being accommodated via straightening of the fibres resulting in fibre separation and then complete fibre pull-out or fibre fracture. The 3D strain fields, computed by Digital Volume Correlation, revealed increasing out-of =plane deformation in samples with decreasing inter-fibre bonding. Further, the relation between the L2 norms of the out-of-lane strain fields with displacement was computed and found to follow a second order polynomial, with an increasing slope in samples with reduced inter-fibre bonding. It was then shown that the accumulated out-of plane deformations could be used as a metric to quantify the relative contribution of inter-fibre bond breakage, and subsequently, fibre pull-out during tensile deformation of handsheets. The results demonstrate that 4D imaging provides new insights into paper deformation mechanisms.