values for the four variables are unknown until the program is run.
That's often true for most variables used in a program.
Why is that a problem for you with your program?
what to put in the brackets
You should define variables that will be given values as the program executes. When they all have values, you can then use the variables as arguments to the constructor:
int first = 123; // give variables values
String secStr = "asdf";
SomeClass sc = new SomeClass(first, secStr); // use variables in a constructor
another way to use the toString method
The toString() method needs to return a String. It's up to the programmer to decide what goes in the String and how to build it.