This technical report describes CHERI ISAv4, the fourth version of the Capability Hardware Enhanced RISC Instructions (CHERI) Instruction-Set Architecture (ISA). CHERI is being developed by SRI International and the University of Cambridge. This design captures four years of research, development, refinement, formal analysis, and testing, and is a substantial enhancement to the ISA version described in UCAM-CL-TR-850. Key improvements lie in tighter C-language integration, and more mature support for software object-capability models; these changes result from experience gained in adapting substantial software stacks to run on prototype hardware. The CHERI instruction set is based on a hybrid capability-system architecture that adds new capability-system primitives to a commodity 64-bit RISC ISA enabling software to efficiently implement fine-grained memory protection and a hardware-software object-capability security model. These extensions support incrementally adoptable, high-performance, formally based, programmer-friendly underpinnings for fine-grained software decomposition and compartmentalization, motivated by and capable of enforcing the principle of least privilege. The CHERI system architecture purposefully addresses known performance and robustness gaps in commodity ISAs that hinder the adoption of more secure programming models centered around the principle of least privilege. To this end, CHERI blends traditional paged virtual memory with a per-address-space capability model that includes capability registers, capability instructions, and tagged memory that have been added to the 64-bit MIPS ISA via a new capability coprocessor. CHERI also learns from the C-language fat-pointer literature: CHERI capabilities can describe not only regions of memory, but can also capture C pointer semantics allowing capabilities to be substituted for pointers in generated code. CHERI’s hybrid system approach, inspired by the Capsicum security model, allows incremental adoption of capability-oriented software design: software implementations that are more robust and resilient can be deployed where they are most needed, while leaving less critical software largely unmodified, but nevertheless suitably constrained to be incapable of having adverse effects. For example, we are focusing conversion efforts on low-level TCB components of the system: separation kernels, hypervisors, operating system kernels, language runtimes, and userspace TCBs such as web browsers. Likewise, we see early-use scenarios (such as data compression, protocol parsing, image processing, and video processing) that relate to particularly high-risk software libraries, which are concentrations of both complex and historically vulnerability-prone code combined with untrustworthy data sources, while leaving containing applications unchanged. This report describes the CHERI Instruction-Set Architecture (ISA) and design, and provides reference documentation and potential memory models, along with their requirements. It also briefly addresses the CHERI system hardware-software architecture, documenting our current thinking on integrating programming languages and operating systems with the CHERI hardware.