A chemically simple four-component model of animal cell plasma membranes can mimic some important properties of the far more complex biological mixture. In the compositional region of the model mixture where co-existing liquid-ordered (Lo) and liquiddisordered (Ld) phases coexist, three interesting behaviors were studied: (1) the first appearance of macroscopic phase domains; (2) the line tension between the coexisting domains; and (3) the bending energy of the coexisting domains. A minimal line tension ~ 0.3 pN is required for macroscopic phase domains to form. A range of line tensions just above this minimal value where phase patterns are observed, so-called “modulated phases”, which seem to result from line tension competing with other interactions. The bending energy of the Lo phase exceeds that of the coexisting Ld phase, but the addition of a transmembrane ?- helical peptide increases the bending energy of the Ld phase but does not significantly affect the liquid ordered phase. Combining these studies sheds some light on the way these energies interact with each other to result in phase separation and modulated phases.