I have a cold solid vascular thyroid nodule that has doubled (now 4cm) in 6 months. had fna, was benign so they are not worried. should it come out?

Probably yes: Any nodule that grows significantly in a short period of time needs to be evaluated for cancer. Consider having another biopsy if it has grown since the biopsy. However, those nodules that are greater than 3 cm should be removed.

Yes: The bigger the thyroid mass, the higher the likelihood of false negative (benign result although cancer still possibly exists) because you could possibly have a fleck of cancer within this 4 cm mass that was not sampled in the biopsy. Since you are very young (32 yo), a 4 cm mass should probably be removed. Otherwise, you should follow this closely and repeat the biopsy if more changes are seen.