Facing the pressing ecological problems caused by urbanization, researchers have tried to seek for an effective solution with urban metabolism theory originated from the analogy of natural ecosystem. The metabolic concept has been introduced into artificial ecosystem and thus shapes the theoretical frameworks of the urban metabolism, industrial metabolism, household metabolism and social metabolism, blazing new trials for the urban metabolic studies. In order to penetrate into the urban metabolism issues, we divided the concept with the generalized and specific senses according to different metabolic objects and boundaries. The Aristotelian causality explores the constituted causes of a thing, embracing the material cause, formal cause, efficient cause, and final cause. A four-causes-map for urban metabolism on the perspective of Aristotle's wisdom was thereby proposed to set up an integrated framework of multi-scale metabolic theories. The urban metabolism studies that have been widely extended and stimulated with myriad innovative methodologies can be interpreted as three main streams in view of Four Causes: 1) Material-based methodology emphasizing the energy and material fluxes of metabolic processes, e.g., material flow analysis,embodied energy (emergy) analysis; 2) Form-based methodology shaping the external morphology and internal structure of urban metabolism, e.g., spatial analysis, ecological footprint and ecological network analysis; 3) Hybrid methodology providing a panoramic figure of urban metabolism. Finally, the future trends and potential solutions were discussed to explore the metabolic pattern and carrying capacity issues relevant to the concrete urban planning activities.