For release highlights of LLVM/Clang 4.0, see our feature overview for the advancements made to this compiler stack over the past half-year. LLVM 4.0 was supposed to ship back in February but bugs had dragged out the release until today.
The official release announcement can be read on llvm-dev. "This release is the result of the community's work over the past six
months, including: use of profile data in ThinLTO, more aggressive aggressive dead code elimination, experimental support for coroutines, experimental AVR target, better GNU ld compatibility and significant performance improvements in LLD, as well as improved optimizations, many bug fixes and more."
Onward to LLVM/Clang 5.0! We have done some Clang 4.0 benchmarks while more compiler benchmarks will be coming up shortly on Phoronix.