Hi,
You need to choose recurrence 'Every hour', and then below it add 4 entries with 15 minutes interval between them, eg. 00m 00s, 15m 00s, 30m 00s and 45m 00s (each entry specifies the run time to start the next workflow execution within the given hour)
Of course, you can use any offset withing the hour instead of 0; for example, the entries could be something like 05m 00s, 20m 00s, 35m 00s and 50m 00s
