1. RAM each new account = 4KB, does it mean that 10000 new users need to consume, 10000 x4KB =39M?
Yes, if you want to «sponsor» the creation of an account for your users, you will need EOS equivalent to 39M. But it’s up to your decision or not whether you want to sponsor them. They will still be able to interact with your contract if they have created their account using some other way.
2. Can RAM be released? How do you release it?
Yes, your smart contract is able to release it if you clear the data. Remember that RAM is used to store the state of your smart contract (which is managed by using the DB API)
3. Does the data store need to buy a hard disk?
Anything you stored in «state» is billed as RAM. Things stored as part of the blockchain log is billed to the transaction signer when the transaction is sent, i.e. NET.
4. how much does the 200000 pen / day deal cost?
It really varies across the different smart contracts. I suggest you estimate the size of the data you want to store in RAM (i.e. the size of the struct you store inside the table multiplied by your expected table size), the size of your action (i.e. the size of your action struct plus the transaction overhead), and measure how long on average your action will take to process.