The design and development of consumer products require an interdisciplinary approach, often constrained by time-consuming prototyping and manual decision-making processes. As product complexity increases and market demands evolve, the need for automation and intelligent collaboration becomes evident. This paper presents a case study on the design and virtual validation of a premium pen using a multi-agent system, leveraging the integration of large language models (LLMs) and software agents. This combination enables a rational representation of human expertise and interactions, streamlining the design process while enhancing adaptability. Using CrewAI, agents were configured with specialized tasks, collaborating to optimize design, select sustainable materials, and establish quality standards. The agents generated a markdown report and a 3D simulation using Blender and Python, ensuring efficient coordination for an ergonomic, sustainable, high-quality pen. By modeling the rational behavior of human experts, the system demonstrated how LLMs and multi-agent coordination can reduce decision overhead and improve collaboration. The results show that multi-agent systems streamline product development by reducing decision overhead, improving task delegation, and enhancing collaboration. The final design met strict virtual quality standards and aligned with market preferences. This study demonstrates the role of multi-agent systems and LLM integration in Industry 4.0, supporting digital prototyping and virtual simulations to replace traditional physical prototyping.