Yes, ext4 uses inode mutexes by default like ext3. They're needed if you want to use a journal, for example. Open http://www.kernel.org/doc/Documentation/filesystems/ext4.txt and look for dioread_lock for more information.

Also, does anyone know what does btrfs do for writes? Its .txt is considerably shorter than ext4's unfortunately.

(I could only find "Btree lock contention: The btree locks, especially on the root block can be very hot. We need to improve this, especially in read mostly workloads." in the unclaimed projects which seems a bit related)