I still haven't gotten around to reading Harbison's book on Modula-3, so it may be a while before I get to M4. I downloaded and did some Modula-3 stuff a while back. M-3 is pretty straightforward though some of the idioms were not obvious. The amazing thing about M-3 was the massive amount of libraries and utilities that accompany the SRC3 download - made the JDK look small.
One interesting snippet from the Modula 4 page was "the proposal for a Haskell-like type system to replace generics." Be interesting to see how that plays out.