unfortunately i am at work and so proper quotation will have to wait until i'm back at home, but i would like to propose that Ishi is the fifth herald that we meet in WoK. and we do so in the prologue. as szeth goes to leave the ballroom that the treaty signing party is being held in, he's accosted by a drunkard who asks him "Have you seen me?"
some have speculated that this could possibly be hoid, though for variuos reasons i don't think this to be the case. drunkeness seems very out of character for hoid. he's a busy man, and wouldn't likely take time out to get sloppy at an alethi party (even though he goes on to be the life of every alethi party). what's more, the question "Have you seen me?" would be a strange one to ask szeth. obviously szeth hasn't. hoid is very careful about what he reveals of himself.
the drunk, iirc (and i will add quotes later), is described as a bald man with a long unkempt beard. Ishi is bald and has a long beard. Ishi being drunk and asking Szeth "Have you seeen me?" makes sense, as the Heralds have each gone insane, and their likenesses are everywhere in the Vorin world. there's a good chance that most everyone has seen him. but of course, Szeth would never expect to find a Herald stinking drunk at a party, and so he dismisses the man.
if this holds up to scrutiny, i think we can add the question "Is Ishi in the WoK prologue?" to the ultimate list.