fl.label1.Text= f.ToString() + g.ToString() ;
this line wont perform the mathematical addition its string addition
for eg:
int f = 10;
int g = 20;
f1.label1.Text = f.ToString() + g.ToString() ;
o/p will be 1020 and not 30
For mathematical addition just add two int variable as below
int f = 10;
int g = 20;
f1.label1.Text = Convert.ToString(f + g);
/EDIT
In your case Form4 f1 = new Form4() is a local variable which has an empty value or a default value for the member labelofform4.Text because you are not updating it.
second you said its not working I dont see any code to display the form4 was is not included for any purpose?
/EDIT