Very good idea, it's just my opinion but I think that 4 and 6 are just a little bit too high though, maybe 2|3, seeing as this will happen every time anything dies? Imagine this with retrovirus, ouch. It would be like plague on steroids even with counters of 2|3. Honestly, I'd say counters of 1|1, stackable with multiple instances of the card would be completely fine for 3|2 cost... 4|6 counters for 2|2 cost would be some of the best creature control imaginable, haha.
Anyways, all that aside, it really is an awesome idea for a new card.
Edit: Ohhh, I think I get it now, 4|6 total poison counters, not 4|6 to everything. In that case, disregard everything I just said except for the first three words of the post, haha.