** Tahajud **time is after Esha Namaz till Fajar..
minimum Raqats 2 hain aur maximum ki koi limit nahi :)..bas Tahajud ki Niyat karni perti hai jitni bhi raqats apko perhni ho..baki Namaz same hai :)..
adding to that ..
Thajjud, you do it in installments of 2 and 2 .. not 4 rakaats together, even if you are doing 4 rakats tahajjud. And the best time is the last 3rd part of the night, the one closer to the Fajr