(1) In the experiment, f is obtained by parallelogram, while F' is obtained by experimental method, and its direction must be the same as that of the rubber band. Due to the inevitable errors in the experimental process, there is a certain deviation between the theoretical value and the experimental value.

(2) In this experiment, the effect of two pulling forces is the same as that of one pulling force, and the scientific method adopted is equivalent substitution method, so C 。

(3) If the thin thread is replaced by a rubber band, the experimental results will not change as long as the nodes are pulled to the same position.

(4) Solution: A. In order to read accurately, the spring must be calibrated to zero before the experiment, so A is correct;

B, the angle between the two components in the experiment is appropriate, which is conducive to reducing the error in drawing, not the bigger the better. If the angle is large and the resultant force is large, the A spring will be pulled out, so B is wrong;

C, when marking the direction of tension, point a point along the direction of the rope. After removing the tension, the connection point O and this point are in the direction of tension, so C is wrong;

D, in the experiment, we must ensure that the rubber band is pulled to the same position, that is, the function of one force is the same as that of two forces, so D is correct.

Therefore: ad.

So the answer is: (1)F? f′(2)C； (3) constant (4)AD