Molecular genetics has been successful in identifying leaf- size regulators such as transcription factors, phytohormones, and signal molecules. Among them, a ROTUNDIFOLIA4-LIKE/DEVIL (RTFL/DVL) family of Arabidopsis, genes encoding peptides with no secretion-signal sequence, is unique in that their overexpressors have a reduced number of leaf cells specifically along the proximodistal axis. However, because the RTFL/DVL lack any obvious homology with functionally identified domains, and because of genetic redundancy among RTFL/DVL, their molecular and developmental roles are unclear. In this study we focused on one member in the family, ROTUNDIFOLIA4 (ROT4), and identified the core functional region within it and we found no proteolytic processing in planta. Developmental analysis of leaf primordia revealed that ROT4 overexpression reduces the meristematic zone size within the leaf blade. Moreover, induced local overexpression demonstrated that ROT4 acts as a regulator of the leaf shape via a change in positional cue along the longitudinal axis. Similarly, ROT4 overexpression results in a protrusion of the main inflorescence stem, again indicating a change in positional cue along the longitudinal axis. These results suggest that ROT4 affects the positional cue and cell proliferation along the body axis.