Really impressive, well done! Must have taken a few practices to get all four flashgun blasts in the 15 sec window!
Not that hard! Set the camera to 5 second timer, wait for the click, fire off the flash using the test button on my wireless triggers, walk around a bit and wait 2 seconds or so for the flash to recharge, then fire it again.
I've only ever tried it with an old Vivitar 285 and with a 30sec timer could only get about four pops out of it. I'll have to try it with my Canon speedlite.