How to change the input action being used according to an int variable?

Switch statement is probably your best bet.

     int randomInt = Random.Range(1,4);
     Vector2 inputVector;
     switch(randomInt)
     {
         case 1:
             inputVector= new Vector2(0,0);
             break;
         case 2:
             inputVector= new Vector2(1,0);
             break;    
         case 3:
             inputVector= new Vector2(0,1);
             break;   
         case 4:
             inputVector= new Vector2(1,1);
             break;
         default:
             inputVector= new Vector2(0,0);
 
 }