Out of courtesy, I would call and downsize your party from 4 to 2. Then the restaurant knows that they can give you a two-top and save the four-top for another party - that may also have downsized say from 6 to 4 people or upsized from 2 to 4 people.
Now if you were upsizing from 2 to 4, it would be a bit more problematic, because they would have to find a second two-top to combine to make a four-top.
While the other poster said they just walked up to the podium and did so, and were met with no problem, the host staff is supposed to be accommodating to all parties with reservations when they can. You don't realize the level of stress that causes when that happens all night long. The staff spends time trying to move around tables to accommodate everyone, if every group that walked up to the podium had more/less people in their party.
However, wouldn't it have been nicer if they knew ahead of time and could have changed the reservation - allowing the restaurant to plan ahead much more easily.
After eating out in restaurants for years, I see the hoops staff go through to accommodate people who might have easily made the adjustments before they left.