let baby sleep at night or wake them up??

Ann marie
Ok I'm a first time mommy and breastfeeding exclusively to my 4 week old baby... I noticed at night she sleeps over 3 hrs almost 4 before waking up to feed... at her two week appointment the dr told me never to let her go past 3 hrs and I should wake her up... well she eats lots before going down at night... almost a cluster feeding for 3 hrs... then she goes into a long sleep pattern which I also sleep and enjoy it! 
She makes lots of dirty diapers is gaining weight...should I wake her up after 3 hrs or wait till she wants to be feed??