It will get better. Just be patient.
I am a mom who practiced CIO and really believes in routine/scheduling BUT... at 4 weeks old I think what you are asking is a bit much to expect. Personally, and this is just me, I couldn't imagine our baby stretching out time in between feedings to more than 3 hours and definitely not sleeping more than 4-6 hours 'through the night' until 12 weeks or later. I remember at that age (4 weeks) we were still waking up every 3 hours to feed him and during the day he barely made it 3 hours in between feedings.
Although I am not a physician, I can only imagine that trying to hold him off or withhold food causes the baby anxiety and stress and could be a factor in his inability to sleep. At that age, the baby's stomach is no bigger than a marble or a ping pong ball.