Following the classical works on the modeling of catalyst layers in fuel cells, refined models on level 3 (L3) incorporating mass transport into L2 models have recently been applied to the CO2 reduction reaction (CO2RR), hydrogen evolution/oxidation reaction (HER/HOR), and oxygen evolution reaction (OER). L3 models yield the local pH and reactant concentration in the diffusion layer (0.1-100 μm), the scale for models to meet experimental measurements. For instance, Monteiro et al. measured the local pH during CO2RR at a distance of 80 μm from catalyst surface, and showed the consistency with numerical simulations. While L3 models are often claimed to be able to calculate local concentrations, it should be emphasized that the term "local" refers here to a macroscopic perspective with a relevant scale of ~100 nm. There is thus a gap to the microscopic reacting zone that is located in the EDL. The EDL is not resolved in L3 models, while recent experimental evidences point to the central role of the EDL in understanding electrolyte effects. Incorporating EDL effects into L3 models to achieve a unified treatment of all components on level 4 (L4) has been attempted in recent works. In this Account, we introduce our approach to this L4 integration challenge. In the next section, we outline the framework of our approach, highlighting important know-how of handling the coupling between different module components. Afterwards, the framework is employed to rationalize potential-dependent Tafel slopes, cation effects, and pH effects that are hot topics of current discussions. Applications will cover ORR, CO2RR, OER, formic acid oxidation reaction (FAOR), and hydrogen peroxide reduction reaction (HPRR). In the end, we share our perspective on how to further the integration of theory and computation in L4 models.