I stopped nursing 4 months ago. is if safe to get a baseline mammogram now ? would my breasts be more sensitive to the radiation ?

Absolutely : Please get one as you are already 45 and the first one is recommended by 40, and at least every 2 years. You will not transfer any radiation to your baby nor will your breasts be more sensitive to the radiation at this time

Yes: Mammograms are easier to read and interpret if there is more fat and less active tissue, so it's best to wait 6 months after stopping breast feeding. Also, breasts may still be more tender, and the procedure could be more uncomfortable if the milk glands were recently active. However, there is no increased risk to you (from the small amount of radiation) due to having recently nursed a baby.

I usually recommend: 6 months to allow the lactational changes to resolve, but 4 months is probably OK. Lactation changes make the breasts more dense, and therefore decrease the sensitivity of mammography. The delay has nothing to do with radiation sensitivity, it is only to allow the breasts to get back to their baseline appearance.