This is how Battery Low notification for DualShock 4 look like. This comes straight from a recent video review of FIFA 14 Playstation 4 version.
As per the details in the screenshot below, the battery low notification for DualShock 4 controller will appear on top right hand side corner along with the user-name and will be in transparent color slightly.
When multiple DualShock 4 controller is used, user-name will be one of the parameter to make out which controller needs recharging, furthermore, there would also be a color indicator i.e lightbar on controller will change when controller needs recharging.
Take a look at the snapshot below showcasing this Battery Low notification.