The Medipix4 chip is the latest member in the Medipix/Timepix family of hybrid pixel detector chips aimed at high -rate spectroscopic X-ray imaging using high -Z materials. It can be tiled on all 4 sides making it ideal for constructing large -area detectors with minimal dead area. The chip is designed to read out a sensor of 320 x 320 pixels with dimensions of 75 gm x 75 gm or 160 x 160 pixels with dimensions of 150 gm x 150 gm. The readout architecture features energy binning of the single photons, which includes charge sharing correction for hits with energy spread over adjacent pixels. This paper presents the specifications, architecture, and circuit implementation of the chip, along with the first electrical measurements.