Drivers have the potential to see your ad a maximum of four times per day. Facebook sets this maximum to ensure the most relevant and recent content is being shown to users. Having a limit of 4 impressions per user per day helps to keep your content relevant without oversaturating your drivers.
This means your ad will only be able to reach a certain amount of drivers per day. So, if your audience has 1,000 drivers, the absolute maximum number of Impressions in one day is 4,000.