First of all, Make 4.0 has integration support for GNU Guile. Guile is the extension system of the GNU project that is a Scheme programming language implementation and now in the Make world will be the embedded extension language.
GNU Make 4.0 also features a new "output-sync" option, "trace-enables" for tracing of targets, a "none" flag for the "debug" argument, and the "job server" and .ONESHELL features are now supported under Microsoft Windows.
GNU Make 4.0 also has new assignment operators, a new function for writing to files, and other enhancements. It's been reported that Make 4.0 also has more than 80 bug-fixes.
More details on GNU Make 4.0 can be found from their release announcement on the mailing list.