Its sometimes hard to know how hard to go in a set of intervals. My guide is firstly to remember that we work in a training zone. So T4 is a range between 96% and 102% of your Anaerobic Threshold. Depends on how you are feeling but if you have a set of 5x1km on the track its best to work into them. Start the first one a bit easier and get to the last one by incrementally getting quicker each one.
Being aware of your targets helps this of course. If you know your threshold pace on the run is 4min/km then start at 4:05 and work towards 3:55.