X-ray picked up 4mm lung nodule, possibly calcified. could it be larger and the rest not visible on an x-ray due to non-calcification?

Very common finding: Here in kansas city, most of us have one of more of these little nodules from having met and contained histoplasmosis. They're nothing to worry about. A lesion wasn't missed because it wasn't calcified -- i've watched radiologists at work and you can trust them. There are lesions that are so unlikely to be cancer they're best left alone. Unless your physician tells you otherwise, just watch it.

Unlikely: Xrays are pretty accurate when it come to measuring nodules. I've been amazed over the years how accurately the radiologist can measure nodule via plain film. I doubt it any larger than 5mm's.