We do not have any official products that are compatible with a Mifare 4k chip in implantable format… however I do have some early prototypes of an xM4 with a legitimate Mifare 4k chip inside… but the chip is large and that meant we had to use thinner walled glass. It turns out the glass was not thick enough to be considered “safe”… during sterilization many of them broke under pressure… however I do have 2 that survived… if you think you’re brave enough, I might consider selling you one with the understanding that the chance of it breaking is much higher than our other x-series products.