reading free internet info (so taken with a grain of salt), one common idea was to know what your maximum possible effort is (be it max heartrate, lactate threshold, max power, max 1 hour power, etc) before you start. then to measure your interval effort based as reaching a certain percentage of one of those things, for a given time.
so some measurement, knowledge of your personal threshold for effort can be pretty helpful.
another common theme was recovery, serious rest between intervals, of a similar duration of the interval.
however, if one hasn't the capacity to check their lactate threshold, one hour power, 20 mile tt sustained effort, etc., they can still probably go by feel. try just going as fast for 3-4 minutes as you can, the go as slow for 2-3 minutes as you can...repeat for an hour or so. warm up well first, and have a decent base so you don't overstress the muscles, and listen to the muscles, don't push cramping muscles, or spasms, until you have a very good feel for what the day after results of those feelings will be.
as Mike T said, intervals are for those who want to get as fast as they can, and not just for 4 minutes at a time, they'll help you sustained tempo speed overall.