This paper presents the design, integration, and validation of ProSSSy-IV, a modular mobile robotic platform developed for service robotics applications in Smart City and healthcare environments. From a systems engineering perspective, the platform adopts a layered architectural approach that clearly separates mechanical actuation, embedded control, and environmental sensing into three hardware levels. The system incorporates ROS-compatible components, including a Raspberry Pi 4 for high-level integration, an Arduino Mega 2560 for motor actuation, and onboard sensors such as RPLIDAR A1 and MPU-9250 IMU. A comprehensive URDF model was developed using Xacro macros, integrating 15 links and 14 joints, with precise definitions of inertial and visual parameters to ensure accurate spatial correspondence in ROS and RViz. The TF frame hierarchy was fully validated and used as the foundation for sensor fusion and SLAM applications. The platform’s behavior was also validated in Gazebo simulation, employing realistic kinematic parameters and velocity controllers. Differential drive models, sensor emulation, and odometry feedback were tested under reproducible conditions, ensuring consistency between simulated and physical performance. By combining modular hardware design, accurate URDF modeling, and realistic simulation workflows, the ProSSSy-IV platform serves as a flexible and reusable testbed for robotics education, prototyping, and deployment in structured indoor environments such as hospitals, laboratories, and smart buildings.