YBa2Cu4O8 is a special member of the Y-based High-Tc cuprate superconductors since it exhibits two CuO chains per unit cell, compared to the single CuO chain per unit cell displayed by the structurally related YBa2Cu3O6+x compounds. Various experimental evidence points towards important differences between the plane-chain interplay of the double-chain and single-chain materials, and a full understanding of the chains remains an crucially important aspect of the multi-phase physics in this prototypical family of High-Tc cuprates. Here we propose to use D33 to further our study of the chain physics in YBa2Cu4O8 from SANS measurements of the vortex lattice. Using a new experimental geometry, we will obtain measurements that are directly sensitive to the temperature-dependent superfluid density along the chain axis. Until now such measurements were complicated since the signal studied was a convolution of supercurrent responses along both a- and b-axes. The proposed measurements will allow more accurate comparison between various models for chain superconductivity, and clarify the mechanism for superconducting chain states in YBa2Cu4O8.