Choosing between Gramercy Tavern, Union Square Cafe, or Gotham Bar and Grill for a very early Saturday dinner (4:00). GT is 1st choice, but how comfortable is 'at the bar'? Any of these better than the others for this particular time?

All excellent choices. You can't go wrong. GT "at the bar" at 4pm should be very comfortable. It's a nice room, not crowded by NYC standards.
All should be dead on a Saturday at 4.

All your dilemmas in life should be so great! You can't go wrong with any of the three.

I have not been to your other two choices, but I have been to Gramercy Tavern in the tavern room at around 4:00 on a Saturday afternoon, and it was wonderful. That seemed to be the perfect time to go, no wait for a table. Very comfortable room, maybe my favorite restaurant.

I've eaten lunch in the bar at Grammercy and it was indeed wonderful, but it is small tables, rather crowded -- it's like, well, eating in a bar. I'm not even sure that Gotham Bar and Grill serves that early on Saturday, but it would actually be my first choice. I think it is even more wonderful than the bar at Grammercy. Comparing it to the main dining room at Grammercy would be a little harder, but I'd definitely put it above the bar at Grammercy.

While I really like Union Square Cafe also, I think either of the other two are actually better.