How to set Local Notification repeat interval to custom time interval?

I am making an iPhone app, which has a requirement of Local Notifications.

In local notifications there is repeatInterval property where we can put the unit repeat intervals for mintute, hour, day,week,year, etc.

I want that repeat interval should be 4 hours.

So every 4 hours the local notification comes.

I dont want the user to set seperate notifications for each.

I want the user to be able to set repeatInterval as 4 hours.

How do I do that?