Hi Lardass,
4 litres is good and if you are comfortable with that it is okay.
I find for myself when SSing I stay around 4/5 litres a day and that is what suits me.
If it is a very hot day you might need to increase your uptake, or if you are working very hard...we all find a level after a few weeks when we feel just right.
Very difficult to compare ourselves with each other, as long as you are drinking that recomended by your programme.
Love Mini xxx