We describe an improved Taylor dispersion method for four-component systems, which we apply to measure the main- and cross-diffusion coefficients in an Aerosol OT water-in-oil microemulsion loaded with one of the reactants of the Belousov-Zhabotinsky (BZ) reaction, water(1)/AOT(2)/R(3)/octane(4) system, where R is malonic acid or ferroin. With [H2O]/[AOT] = 11.8 and volume droplet fraction phi(d) = 0.18, when the microemulsion is below the percolation transition, the cross-diffusion coefficients D-13 and D-23 are large and positive (D-13/D-33 congruent to 14, D-23/D-33 congruent to 3) for malonic acid and large and negative for ferroin (D-13/D-33 congruent to -112, D-23/D-33 congruent to -30) while coefficients D-31 and D-32 are small and negative for malonic acid (D-31/D-33 congruent to -0.01, D-32/D-33 congruent to -0.14) and small and positive for ferroin (D-31/D-33 congruent to 5 x 10(-4), D-32/D-33 congruent to 8 x 10(-3)). These data represent the first direct determination of cross-diffusion effects in a pattern-forming system and of the full matrix of diffusion coefficients for a four-component system. The results should provide a basis for modeling pattern formation in the BZ-AOT system.