AFAIK the diffrence is: IP4, the current IP System supports 32 Bit IPs, therefor there may be 4 billion individual IPs on earth. Obviously this isn't enough anymore since every coffee-machine has its IP these days. So they moved on to 64 Bit IPs with the IP6 protocol that allows ... ehm ... a lot more IPs. I don't think the 4 or 6 in IP4 and IP6 reflects the number of bytes. It's just a version tag.
But, ignorant as I am, maybe I am completely wrong again :P
But, ignorant as I am, maybe I am completely wrong again :P