How many days without pooping is concerning for a 4 week old baby?

Depends. Breast fed babies can poo a little every feed or a big blow out once a week.If soft, it is normal. Most formula fed kids will get into a several daily or every other day pattern.
Depends. Depends upon diet breast vs. Bottle. Also depends upon infant's abdominal size, vomiting, etc. I would take baby to dr though. Have to consider mechanical or hormonal (thyroid) issues.