Multicore fibers have been recently proposed as a possible solution to overcome the problems which affect fiber-based lasers and amplifiers when their output power increases, especially the severe power limitations imposed by thermal effects, responsible of the detrimental phenomenon of mode instability. In this paper the influence of the heat load generated in the Yb-doped cores of four-core fibers on the guiding properties of the supermodes are analyzed. Effective area and single-mode regime are calculated for fibers with different core-to-core separations, in order to identify the most convenient optical coupling regime among the cores in the presence of strong thermal effects. Moreover, through the comparison with dual-core fibers when the same total heat load is generated, useful guidelines for the choice of core number in multicore fiber cross-section are provided. Results show that there is a trade-off between effective area and single-mode regime, because four-core fibers provide the largest effective area, but they become multimode when thermal effects are significant, especially if the cores are optically coupled. When the distance among the cores is large enough, multicore fibers demonstrate to be less affected by the generated heat load, regardless of the number of the doped cores in the transverse section.