There is a general rule that a person who is not obligated cannot do a mitzvah on behalf of someone who is obligated, and according to at least one Rishon this is because a person who is obligated has a stronger Yetzer Hara, and the more difficulty someone has with a Mitzvah the more reward they get.
Since the person not obligated receives less reward, they cannot do it on behalf of someone who's baseline mitzvah receives more.
But what about Tzizit?
A person is not obligated to wear Tzizit unless he wears a four-cornered garment. Therefore he's not Chayiv in the mitzvah until he puts on the four-cornered garment. A person doesn't have the Yetzer Hara to take off the Tzizit from the garment.
So would women, who are not obligated in Tzizit, receive the same amount of reward as men, who are obligated? In this case, since the level of difficulty is the same, wouldn’t the reward be the same?
(I'm not asking if they can put on Tzizit for men just whether it makes sense that the reward level should be the same)