This is an inflection-point release marking the switch from several years of compiler-internals work and cleanup to several years of primarily more applied work like applications and library bindings.
The major user-visible difference from the preceding 4.2.0 release is the addition of support for compile-time compiler-control setting in source files via #DO syntax, in particular
#DO set_control "compiler::verbose_compile_log" "TRUE"; #DO set_control "compiler::trap_int_overflow" "TRUE"; #DO set_control "compiler::check_vector_index_bounds" "FALSE";
For more information on these see (respectively) Pre-Compile Code, Int Overflow Checking and Vector Index Bounds Checking.
This release also marks the introduction of the (currently skeletal) Mythryl benchmark suite, which may be compiled and run by entering make benchmarks at the Linux prompt in the root directory of the source distribution.