If your 4 week old has a bump on his head what do I do
Call your pediatrician with details. He/she will advise you.
Where is the bump and did they hit their head? If at top of head it could just be where bones fused (soft spot).
Wet a small towel, be sure it's warm water and pat it gently on the affected area then you should probably put some oitment. Like cucumber oil, perhaps. Have you heard of that?
If this just appeared and was not there before, take the child to the doctor as soon as possible.