High-end 4-D ultrasound (US) imaging systems typically use probes based on 2-D arrays containing thousands of transducers. Application specific integrated circuits (ASICs), embedded in the probe, manage the numerous transducers, significantly reducing the number of channels needed for the associated scanner. Unfortunately, the closed architecture of these high-end systems prevents research activities, such as the experimental testing of new imaging methods. This article presents the development and application of a novel 4-D US imaging research platform that combines the ULA-OP 256 open scanner and the Adult 4-D Premium transesophageal probe, integrating a rectangular 3072-transducer array. The hardware, software, and firmware developments specifically made on the scanner are described. The functionality of the novel system is demonstrated by comparing simulated and experimental acoustic fields when transmitting focused wide beams (FWBs) and planar diverging waves (PDWs). The latter beams are exploited for high-frame-rate imaging, demonstrating the platform capable of reconstructing, in real time, up to over 800 frames/s over arbitrary planes. In vivo, bi-plane, B-mode images from the carotid artery of a healthy volunteer have been obtained together with dynamic multigate spectral Doppler (MSD) images. The novel platform uniquely integrates the benefits of a programmable open scanner with a high-density 2-D array. By enabling the real-time evaluation of innovative 4-D imaging and Doppler methods, it has the potential to significantly impact applications so far tested with 2-D probes containing a limited number of elements.