The best way to get a good tip as a bartender

is to stop charging customers for their drinks.

If people are not charged for drinks, they feel an obligation to give out some money. Further, you have just done them a service and demonstrated yourself to be a pretty nice guy, so they will give that money to you.

A $4 tip works out well for everyone in that case. You get a bigger tip that normal, they get a cheaper drink. The only one who loses is the bar owner. But fuck him. He makes like 1,000,000,000% profit anyway.