This question consists of two parts:
How to find out what is the writing mode of ext4?
How to configure an existing ext4 (a journaling file system) to do writes to a file in place such that shredding/wiping can be effective.
I'm not sure this accurately answers the question. If a file is open for reading (say, by a shared library with its contents loaded in memory), you really don't want those contents over-written unexpectedly. So, I believe the normal way (ie when data=ordered) is to write the data to disk, update the inodes/metadata, point the directory entry to the new file, but leave the old file in-place and allocated until all references to it are removed (that is, all processes have released any file_open handles).
In that case, shred/wipe might force an fsync, but the old file is still very much active which means it won't get shredded.