Hi Frank,
Real L5 work is hard – if you’re at L5 power for 4 minutes and it still feels easy either your FTP is set too low or your power meter is reading high. I’d track that down first. If you’re confident in both your FTP setting and accuracy of your power meter, the session should feel like you could maybe do one more after your last one…but you sure don’t want to! So, yes, if it still feels easy raise the effort. L5 trains VO2max, and to do that you should be breathing quite hard. Ride hard enough to breathe hard, that should kick in about 90 seconds in and sooner in the later intervals. Don’t go hard enough that you have significant power fade – finish strong.
Coach Dave