I'm pretty sure this is GCC inline assembly, so with that in mind... And note... My GCC inline days are long behind me, but...
stw
The instruction to be executed
r4
Looks like the name of a register
%0
Refers to the list of variables/registers/parameters after the colon
=m
Is a constraint. =m
means constrain operations on %0
to memory writes, if I'm readng the docs correctly.
See here for better explanations of GCC inline assembly.