The Mark 4 VLBI correlator is a station-based system designed to process data from up to 16 stations at an aggregate data rate of 1 Gbps/station. It is compatible with data recorded on Mark 3A, Mark 4, and VLBA tape-based data-acquisition systems, as well as the new disc-based Mark 5 system which also supports both real-time and quasi-real-time data-transmission over high-speed networks. The system incorporates an XF algorithm implemented in a custom-VLSI chip that can be flexibly configured to cross-correlate VLBI data from one to four baselines of a single 2-bits/sample channel at 64 Mbps/channel/station. Each chip also incorporates integral phase generators, rotators and vernier-delay management circuitry. Each of the sixteen Correlator Boards in the system contains 32 correlator chips and can process up to 8192 complex lags that can be flexibly traded off between baselines, lags and channels. Innovative algorithms allow station-model information to be injected periodically into the data stream from each channel of each station to drive the correlation process. The correlator control software allows up to four independent multistation scans to be processed simultaneously to significantly improve processing efficiency.