4 month old baby boy feet and hands turning purple what causes this and or is this anything to be worried about?

This sounds like cyanosis (if it's just the hands and feet which are affected, we call this periferal cyanosis). Blood flows away from the heart in arteries carrying oxygen around the body. If there are only low levels of oxygen left in the blood by the time it reaches the extremeties (the hands and feet) it looks a bluish colour. There are lots of things that can cause this, so it's important to get a doctor to look over your baby as soon as possible just to make sure it isn't anything serious.