_setupDecimals() -- undefined identifier

I'm trying to specify the amount of decimals of a token. This will cause a compilation error:

import {ERC20} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract MyToken is ERC20 {
   //........

   constructor() ERC20("my token", "a")  {
       // _mint(..... )
     _setupDecimals(4); // !!!
 }

//.........

What's the matter?