> If I put everything into one big cma, then I have to recompile it every
> small change. It takes so long time, that it would make no sense to use
> the interpreter at all.

in case you're doing so this way:
ocamlc -o my.cma mod1.ml mod2.ml mod3.ml mod4.ml

this will recompile everything,
but you can use in your makefile:
%.cmo: %.ml
        ocamlc.opt -c $<
my.cma: mod1.cmo mod2.cmo mod3.cmo mod4.cmo
        ocamlc.opt -a -o $@ $^

then you don't recompile everything, only the modified module

if there are dependencies between modules, just add additional rules:
modfoo.cmo: modfoo.ml modbar.cmi