Some of the potassium in your cells is radioactive and decaying right now at a rate of 4900 decays per second[0]. That's about as close as you can get to the source, and it's not a big deal.
You ignore the fact that the radioactive potassium will be distributed more or less evenly over your body.
Secondly, and more importantly, you bring up this number of 4900 decays per second as if it means anything by itself. Those 4900 decays per second are caused by the presence of 18 milligram of Potassium-40 in your body. One of the elements released by Fukushima is Iodine-131. If you ingest 1 microgram of Iodine-131, this speck will generate 4,6 BILLION decays per second. So yeah, you're right that 4900 decays per second is not a big deal. But then you conveniently forgot the fact that the material released by Fukushima is orders of magnitude more radioactive than Potassium 40.
Secondly, and more importantly, you bring up this number of 4900 decays per second as if it means anything by itself. Those 4900 decays per second are caused by the presence of 18 milligram of Potassium-40 in your body. One of the elements released by Fukushima is Iodine-131. If you ingest 1 microgram of Iodine-131, this speck will generate 4,6 BILLION decays per second. So yeah, you're right that 4900 decays per second is not a big deal. But then you conveniently forgot the fact that the material released by Fukushima is orders of magnitude more radioactive than Potassium 40.