The FST-951H-IV is indeed a notable upgrade from the FST-851H. One key improvement is its enhanced sensitivity to smoke detection, providing quicker response times in detecting potential fires. Additionally, the FST-951H-IV incorporates advanced algorithms for better false alarm rejection, reducing unnecessary disruptions. Its improved design also allows for easier installation and maintenance. Overall, the FST-951H-IV offers superior performance and reliability compared to its predecessor, making it a solid choice for fire detection systems.