So, if sth. gets stolen and I need 4 days for preparation to get my stuff back and by accident the thief moves my airtags and they play an audible sound...There should be a setting to turn off such behavior.
No, you're mixing up two features. The audible sound starts doing its job after 3 days (not 4), that's part one. The second, and in my opinion most important part, is the mere fact that your AirTags will report back their location to you. So worst case that was their last location when they were still in line-of-sight with your iPhone. Best case scenario would be that it is in range of other iPhones, leveraging the Find My network using BTLE and having those iPhones of random other people report your AirTags location back to you, including time stamp like "15 minutes ago".
So no, you don't have to wait 3 or 4 days at all. It's just that in case of lost items, it'll play a sound after 3 days and it might help someone finding your stuff with that AirTag on/in and as with most Apple products, in a few years they'll probably become such a commodity, even my mother would know what to do with it.
Of course, in case of theft, the thief will just throw out the AirTag and keep the stuff attached to fit, and that's why it's only providing an audible sound after 3 days. You wouldn't want a thief to know there's an AirTag inside your backpack minutes after you've lost it. That's a guarantee he'll find where the sound is coming from and either dump it all or just the AirTag (whatever's more convenient). However... let's say your bag gets stolen in plain sight, then you can run after the thief or if you're not willing to put yourself in possible danger, you can grab your iPhone (if that wasn't in your backpack) and let it play a sound, there's a chance the thief will find himself too exposed in let's say a mall or something and just let go of the bag.
Is 3 days optimal? I don't know. Should it be left to the users? Yes and no. I feel it should definitely NOT be left to users freely, as in: choose anything between 1 minute and 1 year. There has to be a common ground that everyone knows off, for example that it will never start beeping within the first day of "losing" it. So maybe Apple should let us choose between like 1 and 7 days, but I could also understand why they wouldn't, sometimes for the general public (because that's what you're partly counting on in these scenarios) it's better to just have a fixed number of days that everyone at some point will know of.