Issues while trying to make an ISO from a DVD in macOS

Question

I have already read most of the questions and answers relating to this issue, which is supposed to be simple, but I still can't do it.

The unmount step goes ok:

sudo diskutil unmountDisk /dev/disk4
Unmount of all volumes on disk4 was successful

but the dd command exits with error:

sudo dd if=/dev/disk4 of=mydisk.iso
dd: /dev/disk4: Permission denied
10656+0 records in
10656+0 records out
5455872 bytes transferred in 2.834248 secs (1924980 bytes/sec)

I can't understand the error dd: /dev/disk4: Permission denied. I can't find anything wrong with /dev/disk4, the DVD plays rather well and I can watch the movie.

ls -l /dev/disk4
br-xr-----  1 erothe  operator  0x1000014 Nov 27 18:27 /dev/disk4

This steps are based on Mac OS X: Best Way to Make an ISO from a CD or DVD.

Notes:

1. if I use diskutil unmount on step 1, the result does not change;
2. I have chmod u+x /dev/disk4 just in case some witches were flying around;
3. the dd command creates a file with 5MB (DVD has 700MB).