horcruxes - how do we know there are only four left?

[allies426]

No: HPFGUIDX 136125

Horcrux recap:

1.  Riddle's diary (destroyed)
2.  Slytherin's ring (destroyed)
3.  Locket - missing
4.  Cup - never located
5.  Ravenclaw/Gryffindor artifact - unknown (Hat, Sword, Phoenix?)
6.  Nagini (suspected) - whereabouts unknown
7.  Piece that is still within Voldemort
(NO WAY can one of the Horcrux's be Harry, IMO, why try to kill him 
so many times if he were a horcrux???)
 
However, we are also told that Voldemort was FURIOUS when he found 
out what Lucius Malfoy did with his diary and that it was 
destroyed.  That would leave only 6 soul fragments behind, not 
seven.  (Incidentally, did anyone notice that the Wizard of the 
Month on JKR's website is the witch who discovered the magical 
properties of the number 7?)  

So after his Horcruxes were reduced in number, what if Voldemort 
created another one, to re-create seven?  Any offscreen murder could 
have been used to do it.  I can envision a scenario where a 
triumphant Harry confronts Voldemort, thinking he has destroyed all 
of the Horcruxes, only for Voldemort to say, "See, you stupid boy?  
Here is my 7th Horcrux, the crumple-horned snorkack of Merlin!" (or 
something similarly obscure and unpredictable)

Allie