Linux lsblk output

sr0 is your optical device, i.e. the dvd drive. There is a 1 under the RM because it is a removable drive. It has a 0 under the RO column because it is not a read-only device. It is of type rom because it is not a disk, but a DVD/CD-ROM device, i.e., an optical device.

sda4 is a 1k partition because it is only a shell of a partition. Old-fashioned disks, i.e., those with ms-dos-type partition table (the modern counterpart is called GPT) could have only 4 primary partitions. Primary partitions are essential to some OSes (older versions of Windows) because they can only boot from a primary partiton. But the need for more partitions has led to the transformation of a primary partition into an extended partition, i.e. one that can be further subdivided into 64 (I think) more logical partitions. Generally, the extended partiton is the last primary partition (sda4), and it is never directly used: in its stead, the first of the logical partitions is created, so that the possibility of creating more logical partitions at a later time is preserved. The 1K you see thus contains only partitioning information, nothing more.

Major:minor are the names with which the kernel refers internally to devices. The first number identifies to the kernel the device driver it must use to communicate with the device. So, if for instance you had several SCSI disks, they would all have major number 8. If you had IDE disks, their major number would be 3, the optical drives are all 11 and floppies are 2. The minor number identifies the specific device among all those using the same device driver: in your case, partitions 1-5 have minor numbers 1-5. If you had a second SCSI disk, their minor numbers would start at 16. These numbers used to be carved in stone, i.e., ship with distros, but with the arrival of udev they are not even required to be globally unique, and may vary between boots. Still, it is most common for them to adhere to the old habit of having 3 for IDE disks, 8 for SCSI disks.