This paper comparatively analyses the current models far Software Process Assessment and Improvement (SPA/SPI) e.g., SPICE, CMM, ISO 9000 and BOOTSTRAP. The configuration of the sample spaces and the partially overlapped fundamental process activities defined in the four models are quantitatively analysed and contrasted. The features, orientation, profile and correlation in the software process capability determination frameworks of the current models are comparatively evaluated. The analysis results provide the software development organisations a complete and objective view of the strengths, weaknesses and interrelationships of these four SPA/SPI models, so the most appropriate models) can be chosen for their crucial long-term SPA/SPI pursuit.