The ST4 was basically an ST2 chassis and bodywork with a 916cc eight-valve desmodromic engine fitted. That engine installation was straightforward, although the front cylinder head was modified from the standard 916 Biposto design in order to fit the motor into the ST2 frame. The exhaust camshaft was moved down in the head by 10mm (0.3in), allowing the larger engine to fit.
For 2000, the ST4 was modified with several uprated parts. A wider rear wheel took a larger 180-section tyre, and the brakes and dash were uprated over the ST2 parts. A new sidestand design replaced the previous flip-up design and there were some detail engine changes, mostly to improve reliability.