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(1) Background: Oaks have achieved notoriety for sufficient levels of sympatric species richness allowing hybridization, thus generating substantial phenotypic variation. Leaf fluctuation asymmetry is an important attribute, as it reflects not only genetic variability but also species buffering capacity. (2) Methods: We investigated the phenotypic diversity of four-oak species (<i>Quercus acutissima</i>, <i>Q. variabilis</i>, <i>Q. fabri</i>, and <i>Q. serrata</i> var. <i>brevipetiolata</i>) using leaf geometric morphometric analysis. Eight leaf morphological indicators (length, width, perimeter, area, left and right areas, areal ratio, and normalized symmetry index) were used to determine the hybridization level, whereas bilateral symmetry indicators were used to assess species environmental adaptation; (3) Results: Phenotypic variation ranged from 1.54 to 29.35 folds and significantly diverged among the studied species. Taxonomically species in Section <i>Quercus</i> (<i>Q. fabri</i> and <i>Q. serrata</i> var. <i>brevipetiolata</i>) are lower than those in Section <i>Cerris</i> (<i>Q. acutissima</i> and <i>Q. variabilis</i>) with good bilateral symmetry. The bilateral symmetry index of <i>Q. variabilis</i> had a larger range of variation, indicating better environmental adaptability; (4) Conclusions: We presume that species in Section <i>Quercus</i> with less leaf fluctuation asymmetry have a high level of genetic heterozygosity; however, this assumption requires further verification. The observed phenotypic diversity reflects a combination of environmental and genetic factors.