help with deed in "seat of the great goblin" instance please

i am doing (or rather trying to do) the deed to discover the variouse murals inside the instance. i have all but the fourth done. wiki says:

"The forth Goblin-mural can be found on the right-side walkway that opens up after returning from the pit."

but, the only door that opens up is after the big troll boss in the pit - the path that leads to a meaningless lake - and there is a mural there but it is the third and not the fourth as i have discovered this already.

i have walked and re-walked the entire instance ... HELP?!?

thank-you