Alfalfa (Medicago sativa L.) exhibits high feeding value in agricultural and livestock production, with its above-ground parts determining yield and quality. Promoting the development of molecular breeding of alfalfa helps to clarify the function of MsARG4 in regulating its growth. Obtaining MsARG4-overexpressing and RNA interference-positive plants via genetic transformation, we compared their phenotypic, nutritional, physicochemical, and cellular structural characteristics with those of wild-type plants. The results showed that the plant height, stem diameter, number of branches, number of lateral branches, internode length, and crude protein content of overexpressing plants were significantly higher than those of wild-type and RNAi plants (p < 0.05). Furthermore, we observed that the cell area of OE lines was larger than that of WT and RNAi lines. The stem-to-leaf ratio of overexpressing plants in the branching and budding stages was significantly lower than in wild-type and RNAi plants (p < 0.05). Our study suggests that MsARG4 promotes alfalfa quality improvement and rapid stem growth.