Figure out how fast your competition or category go. For example, if your competition is averaging 32 mph for 4 km, then you need to be able to go that fast. Of course, if you are starting out, you won't be able to hold that speed.
Try to hold 32 mph for 1 km to start. Start out with something like one set of 4 x 1km at 32 mph, then work up to more sets of 4x1 km. Other strategies include: Decreased recovery between sets, hold 32 mph for 2+ km, stick with 1km sets but increase speed to 33+ mph, etc. Periodically, do the full 4km when fully rested to assess your progress, say once every 4 weeks.
Keep your cadence between 100-110 rpm
Good luck!!!