Can you put a 4 week old in a seated position?

For a moment or two. One can hold a baby in almost any position for pictures or for any other reason, if just for a short time... as long as the baby is comfortable, is being watched, and can breathe fine. A young baby would just flop over if left alone in a seated position... which means there is no reason to leave him in a seated position. Infant car seats carry young babies in a reclining position (not seated).
Can but should'nt. While some 'chunky' neonates may be made to sit, due to their larger body size, normal development for an infant would allow them to sit on their own around 7 months, give and take a few weeks.