Block copolymers are macromolecules formed by covalently joining two or more distinct polymer blocks that may be thermodynamically incompatible. The incompatibility drives segregation of the individual blocks on the molecular scale (5 – 100 nm), producing extraordinarily varied and complex morphologies. This thesis describes the synthesis and phase behavior characterization of tetrablock terpolymers composed of poly(styrene) (S), poly(isoprene) (I), and poly(ethylene oxide) (O) with an emphasis on ABAC-type polymers. Motivated by SCFT calculations, investigation into the phase behavior of sphere-forming SIS′O tetrablocks led to the identification of multiple ordered structures upon varying the symmetry parameter τ = NS/(NS + NS′), where N is the block degree of polymerization. Complementary data from dynamic mechanical spectroscopy, small angle X-ray scattering, and transmission electron microscopy yielded evidence for nine different spherical phases: FCC, HCP, BCC, rhombohedral (tentative), liquid-like packing, dodecagonal quasicrystal, and Frank–Kasper σ and A15, and simple hexagonal packing (HEXS). Close to the order-disorder transition, equilibrium morphologies are formed due to facile chain exchange between micelles. Transition to non-equilibrium behavior occurred several tens of degrees below the order-disorder transition where increased segregation strength between the O core and SIS′ corona arrests chain exchange between domains. Structure and thermodynamic stability of the HEXS phase were examined in greater detail and the phase was found to be especially stable in low-τ samples. Switching the block sequencing from SISO to ISIO led to an extinguishment in complex behavior as only BCC and hexagonally packed cylinders (HEXC) were identified as ordered phases. The decrease in morphological complexity was attributed to the formation of frustrated interfaces as the ISIO molecular architecture mandates contact between the most thermodynamically incompatible I and O blocks. Additionally, synthetic strategies capable of producing ABCA′-type tetrablocks with asymmetrically sized corona chains were developed. These results expand the monomer toolkit capable of producing new types of block polymers and provide a deeper glimpse into the fundamental principles that guide block polymer phase behavior.