Creating C++ classes and objects
First of all, you should avoid "getters" and "setters" whenever possible.  This is because they can be bad for encapsulation as they reveal the internal members.  This would be an ideal example if you were merely trying to understand getters and setters, but it's still not the best approach to learning how classes should work, especially when you start using them for more complex programs.

You should become aware of object construction.  That is, when you first create an object to hold some data at the start, it should be given to that object upon construction.

Instead of using setters, you should construct the object after getting the input:

X testObject(lNum_1, lNum_2, lNum_3, lNum_4);


This will construct the object right away with the initial values.  However, you'll first need to provide a constructor that will handle this.

You can use an initializer list:

X::X(int num_1, int num_2, int num_3, int num_4)
    : num_1(num_1)
    , num_2(num_2)
    , num_3(num_3)
    , num_4(num_4)
{}


As for the getter, it would only be useful if you need the value.  If you just need to display the members, then you can create a display() function:

void X::display() const
{
    std::cout &lt;&lt; "num_1: " &lt;&lt; num_1 &lt;&lt; "\n";
    std::cout &lt;&lt; "num_2: " &lt;&lt; num_2 &lt;&lt; "\n";
    std::cout &lt;&lt; "num_3: " &lt;&lt; num_3 &lt;&lt; "\n";
    std::cout &lt;&lt; "num_4: " &lt;&lt; num_4 &lt;&lt; "\n";
}


(Notice that I've made this function const.  Since it only displays data members and doesn't modify them, adding this keyword will prevent any accidental modifications.)

It would then be called by itself, rather than calling the getter for each member:

testObject.display();


There's much that can be said about all of this, but I've given you a basic idea on where to start.

Miscellaneous:


Try not to get into the habit of using using namespace std.  If you start writing larger programs, you may run into name-clashing issues if this is misused.  More info about that here.
Since you're not using &lt;cmath&gt;, it should just be removed.
_tmain() should just be main(); the former does not exist in C++.
You could leave out the command line parameters if you're not going to use them.

Otherwise, you could certainly replace your input with command line arguments:

./program_name 1 2 3 4


If you do this, you should also be sure to check the value of argc so that the program won't try to run with too few arguments:

if (argc &lt; 5)
{
    std::cerr &lt;&lt; "You must input four numbers!";
    return EXIT_FAILURE;
}


The program name is counted as an argument, so it should be 5.