This data isn't particularly recent, and I don't remember exactly what I did for this, so I'm trying to avoid focusing on that until this is a little bit more refined.
Except EC4 is completely backwards compatible with EC3 (existing JavaScript). That is, a valid EC3 program is a valid EC4 program. So it's not like moving from Fortran to C++. It's list keeping all the Fortran you already have and want, but being able to also link some C++ into the final executable.
This data isn't particularly recent, and I don't remember exactly what I did for this, so I'm trying to avoid focusing on that until this is a little bit more refined.