[plt-scheme] linear update procedures in v4

hello,
srfi 1 allows, but doesn't require, its linear update procedures to be
destructive. So I was wondering, if I use append! in a version 4 #lang scheme
module, is it 'silently' calling the non-destructive append instead?
As you can see, I'm a bit confused about the mutation-related changes
in the latest versions of plt-scheme.
regards,
praimon