Is this behaviour useful? I mean are flags with already 4 additional flags already sufficiently handled or is it useful to add a fifth or maybe a eights flag?
Yes, it's useful. Flagging a post 4, 6, 10, or even 94 times does not mean that anything is "handled". It's not handled until either you take action to correct the problem yourself (such as editing or voting to close), or a moderator sees the flag and takes the appropriate action. So adding your own flags of agreement (or disagreement!) is always useful.
Of course, how you choose to distribute your daily allotment of flags is, as always, a matter of personal preference. If there are some flagged posts that have received relatively little attention from the community, and you want to maximize the likelihood that these flags will be processed rapidly, then adding a confirmatory flag of your own makes good sense.
Additionally, if you're known by the system to be a consistently helpful flagger, then things you flag will be given more weight and shown higher in the queue. It's basically a trust system—if we have reason to trust your judgment based on past history, then we'll give your complaints more attention. If you've been known to repeatedly cry wolf, we tend to postpone handling your flags to a time when we're less busy.
If it is no difference for a mod if a question has 4 or 6 flags it would maybe make sense to remove flags from the 10k queue, when they made it to the mod queue.
Piling on flags definitely makes a difference for moderators. If a particular post has been flagged by 8–10 different users, that's a much stronger mandate than if it has been flagged by 3–4 different users. That, in turn, reduces the amount of time mods have to spend gawking at each flag. If it's been well-vetted by the community, a quick confirmatory glance is all that's required.