The COA/DOB has no effect on the quality of the knife itself. If you liked it before, you should still like it now.
However, if you paid more for the knife thinking that it was a Tanto III, you'd have a bone to pick w/the seller if s/he represented it to you as a Tanto III and you relied on that representation in making the decision to buy it.
In this case, you could ask to return the knife for a full refund or ask for a partial refund of the price based on the difference in the value between a III or IV based on a material misrepresentation or mistake made by the seller but. if the seller is unwilling to agree to either option, you're only recourse would be to take him/her to court for a financial recovery.
I don't know anything about the value of a CR Tanto III vs IV but I'm pretty sure that any difference in price would not be worth your time/$ to file a civil complaint to recover any financial loss resulting from the difference.
So, it is what it is. If you paid too much $ for the knife thinking that it was something it's not, then it's a lesson learned. Move on and forget about it but, if you didn't really lose anything because of this discovery, then it really doesn't matter.
Just don't continue to represent the knife as a III to others, especially potential buyers, knowing now that it's a IV.