i read about pins 11, 12, 13 and i saw that they can be shared.
but can pin 4 and pin 10 (rispectively cs of SD and Ethernet) be active at the same time?
Yes. Pin 4 is the standard chipselect for SD, and pin 10 is usually nominated as OUTPUT as part of the SD operation. This does not interfere with Ethernet operation, what you propose is normal.
i'm trying to do a sketch to send a mail to a email address with attached a .csv file (the .csv file is a database of DHT11 misurations in the SD).
have you ever done something like this?
I have only sent .CSVs of DS18B20 readings to phone via bluetooth. I have only used Ethernet for IoT, which is a hell of a lot simpler than what you are doing, but what you want is possible and I'm sure there is a swag of information about it round here.