The HardSID 4U is a unique combination of a hardware synthesizer and a VST plugin built around the iconic SID chip, known from the Commodore 64 computer. This unit supports up to four SID chips of any version, allowing rich combinations of sound generation. Its design enables musicians and producers to access authentic SID sounds with modern flexibility, bridging vintage hardware with contemporary digital audio workstations.

The VSTi plugin offers advanced features including an 8000Hz update rate on all SID registers and a Wave-in pin for routing external signals into the SID registers, a capability not found in other SID synthesizers. Users benefit from full automation of all parameters, extensive modulation options with an 8x8 matrix, multiple LFOs and envelopes, and a table editor for high-speed value sequencing. The plugin also allows up to four instances to run simultaneously, each corresponding to one SID chip, supporting complex and layered sound design.

On the hardware side, the HardSID 4U is equipped with two CPUs, main memory, and a USB connection designed for low-latency audio performance compatible with USB 1.1 and 2.0. It supports firmware updates via USB and features multiple audio connectors including four mono dry outputs, a mixed stereo output, and inputs for external sources filtered through the SID's analog filters. This setup ensures high-quality sound output and seamless integration with both desktops and notebooks.