Cleave, the HR zones I use were created as percentages of LT, and come from the Australia Institute of Science (what the tester uses), which does a lot of research in the area. Compared to these, Friel's zones will push you to a higher HR, which is probably why you experience what you do in z4. From my reading, combining someone elses exercises with Friel zones is a great way to overtrain.
I'm also including what my zones are as a % of my MHR, which is 188. My LT last test was 161:
.....................................%LT.........%Max
1....Recovery.......................0...75......0...64
2....Aerobic Endurance.............76...91.....65...78
3....Intensive Aerobic Endurance...91...96.....78...82
4....Threshold.....................97..102.....83...87
5a...Anaerobic Endurance..........102..107.....88...92
5b...Power/VO2 Development........108..117.....93..100
5c...Neuromuscular Power
I go on and on about this stuff, because that is the best way for me to gain a decent understanding of it, and so Hermes can correct me if I have it wrong.