Son was diagnosed with a 4 mm ventricular septal defect one day after birth. how worried as a parent should i be going forward.

Depends: They often close spontaneously. When they don't, they can be repaired. Don't worry. You're living in the 21st century!

I wouldn't: VSD's represent an incomplete formation of the wall between the lower chambers.Thankfully, the small ones like you mention usually close by themselves as the wall formation finishes (just late) within the first 6 months.There are so many other issues you will need to address that I would put this on the shelf and not worry about it at all.Do ask about it during checkups.