Whether it is normal or not certainly depends on a ton of stuff.
For instance, if it is on a cpanel server, cpanel will throttle the backups until the load decreases. This can make backups take a long time.
Maybe he is using a tar command or some sort of super bzip2 compression, maybe over iscsi? If kteo tells us how the backups are being performed, we'll be able to tell more, and see if it is normal or where there might be a fault.
This is disk to disk on the same server, so we're really only looking at compression or not, i/o and any potential waits for load issues if the backup system is set to only run when the load is low. Still, from start to completion, 4+ hours is not really normal even at a low priority. In other words, there would have to be some significant loads caused by something else to push it back that far on the processing list.
I just don't see that being a possible normal circumstance if it came down to those variables involved. Sure, it can happen and it doesn't mean something is "broken" or "wrong", but I hope that's not the normal experience people have had, else their systems surely need to be better monitored or better configured (or it's time to upgrade to a system with better specs), which can all be attributed to other causes for it to be delayed (and those are what need to then be dealt with).