I accidently transferred BTC to a non existing wallet. The wallet is now being created and I have to activate and verify it. To activate and verify I have to deposit 4,9 BTC. Is that how it works? Why do I have to deposit so much BTC? There are already 7,3 BTC on that wallet when it is created.
You are almost certainly being scammed.
Bitcoin wallets do not require activation.
In any case, even if you were dealing with some entity that does require verification for using their services, the amount you claim is stored with them exceeds the amount they are asking for - a sane, legitimate business would just charge the amount against what they already have and send you the balance.
Do not send any more funds to these people.
You are being scammed! Don't send any BTC to them.
Proper Bitcoin wallets are generated on your computer / phone, they contain bunch of private keys, each of one has corresponding address, to which you or others can send BTC. There is no concept of "activation" in Bitcoin.
If you accidentally transferred BTC to a wrong address, either it's from somebody else's wallet, then he can send you BTC back (if he wants to), or, it is possible to send BTC to addresses to which nobody has private keys and such bitcoins are most likely gone forever, it is practically impossible to recover them.
I accidently transferred BTC to a not excisting wallet
If you did that, the money would be gone forever and no-one would ever be able to gain control over it.
Bitcoin wallets make special checks to prevent this. Bitcoin-addresses have checksums. It is very hard to accidentally create a Bitcoin-address with a valid checksum.
The wallet is now being created
Bitcoin transactions involve sending money to a Bitcoin-address. These addresses are created by wallets from a private-key. It is not possible to create a private-key from an address. If it were possible, the whole Bitcoin system worldwide would be destroyed and all Bitcoin valueless.
Most likely you were tricked into sending money to an address created by a criminal.
To activate and verify I have to deposit 4,9 BTC. Is that how it works?
No that is not how it works.
It is completely free to download Bitcoin-wallet software to your computer or to your phone and to install and run that wallet software. There are no charges for doing this.
Why do I have to deposit so much BTC.
The thief wants your money and so wants to convince you that this is necessary. The thief is exploiting your lack of knowledge of how Bitcoin really works - in order to steal money from you.
There are already 7,3 BTC on that wallet when it is created
It is almost certain that this money is gone and that you will never get control over it. You can, and should, report this crime to your local police. Awareness of the extent of this sort of crime can help guide allocation of police resources. Do not expect them to be able to recover your money though - it is most likely in the control of someone in a far away country.