As in, MIFARE Classic 4k? There is the flexUG4. It is not available as an x-series (one of the injectable implants). At present, it is highly unlikely mostly due to the insane costs involved in acquiring the chips in the proper format–you can’t buy anything but a full wafer typically. And, unfortunately, that is prohibitively expensive (if we can even find one for these grey market chips). Sorry.
On the upside, the flexUG4 was just re-released and its range is nothing short of awesome!