If customers want to sit at the bar, and theres availability, let them sit down. The environment at the bar is always different than sitting at a table, as is the service. If the place is super busy, I know i've done the 2 people sit, 2 people stand move and hang out.
Ive done 4 at a bar before, but you gotta either be on the corner, which is perfect for four, or it's your only option. 3 max.