The AMBA 4 specifications introduced more interface protocols on top of the AMBA 3 specifications, including ACE, the AXI Coherency Extensions. It addresses high-bandwidth, high-clock-frequency system designs and includes features that make it suitable for high-speed interconnect, typical in mobile and consumer applications.
AMBA 3 is an older generation of AMBA which introduced the Advanced eXtensible Interface (AMBA AXI) protocol. It also introduced the Advanced Trace Buffer (ATB) and extended the APB and AHB protocols.
AMBA 2 is an older generation of the on-chip bus architecture that added the AMBA High-performance Bus (AHB), which is a single clock-edge protocol. It has been widely used on Arm7, Arm9, and Arm Cortex-M designs.