having broke 4 minutes i dont beleive that the 10 400s was the key to it, you are putting too much emphasis on one session, however what it does do is get your body to run at the desired speed in a relaxed manner, this is why some of you have said you could do the session but not the mile, the "5k guys" could maybe do the session but never feel relaxed, what im trying to say it is the manner in which the session is done, that 'will indicate if your in sub four shape. 10 400s is not a balls to the wall session. which is also why you can often run the last few faster.
to break 4 mins it first takes a good winter.
plenty of speed endurance.
a great session for this for me was...
4/5 x 1200m 3 mins rec
or a ladder 1600 , 1200.......
hard hills long and short.......
these were the sessions that helped me break the 4 min barrier and run 10 400s under 60
the hard work was in the bag already any one breaking 4 could do so without running that sesion but not without the hard speed endurance stuff.
i also think that the session isnt the best for getting you ready to break 4 as i liked to run something a little faster than race pace for a few weeks. 400s witha little longer rest or maybe 300s of 100 jog which i loved.
any way all the best.