There seems to be no real right or wrong with this, if you look back through previous threads
http://ukbouldering.com/board/index.php/topic,18073.0.html](e.g.)[/url] you'll find different, advice all of which seems to work. I found
this comment/advice quite informative.
Re your question, 15 mins between sets of four sounds excessive for five minutes climbing, try reducing that to start with, you'll probably find you can get it down quite steeply and still just about scrape through but it really depends on current fitness, intensity of the circuit, etc.
With no basis or science involved I start off taking the amount of time the whole set of four reps plus rest has taken, so 7x1:15 in your case and aim to get it down to just the amount of work undertaken in the set, so 4x1:15. Then when I nearly manage to complete the whole 16 sets, it's just a matter of increasing the difficulty or duration of the laps or the decreasing the rest further depending on aims/time available etc.