As far as I can tell, if you didn't order direct from the ZO site, a few of the in-game bonuses related to the IE (like the pet) will not apply until 4/4.
Yes, there is the difference between pre-order from retailers (where you can change your mind about which edition you buy right up until you open the box) and pre-purchase from ZOS where you have actually paid for the edition. I expect the actual time you will get the pets and other neat stuff is when you actually register the game key after 4/4.