The 4 x 400m means run for 400m as fast as you can then rest...either by monitoring heart beat (til gone back to normal) or walking for few mins til you feel better
In reality you can only do accurate stuff like this on a track...but you could do a good guesstimate....the basic idea is to have a SHORT session (20-30 min max) and have a few fast bursts, recover by walking for a set time between these bursts
a guesstimate of the 4 x 400m would be 4 x just less than half a km
Popsy you can run 1k fast in 5mins
so 4 x 400m means run 2 mins as fast as you can then walk for 2 mins...repeat this 4 times if you do a 5 min jog warm up and a 5 min cool down you have a 26min session
With speed sessions make it up as you go along
start with 5 to 10 min warm up jog
examples are: pyramids 1,2,3,3,2,1 fast bursts with 2 min walking between (as you get fitter you can reduce the walking time between), 4 x 2 min fast with 2 min rest intervals, 3 X 3 min fast etc etc
Fartlek means totally random...make it up as you go along just alternate fast and short bursts eg between lampposts etc
HTH