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.
You don't.
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.
who or what said that you have to deposit BTC into a wallet before being able to use it ? please clarify and be careful it seem like a classic scam – Saxtheowl – 2019-10-13T15:40:43.970
Did someone ask you to transfer money to this address and then later claim that it was the wrong one? Where did the address you transferred to come from in the first place? – David Schwartz – 2019-10-16T15:39:44.300