i guess you are aware that they didn't tell you the truth. the shape of the front pane doesn't have anything to do with whether a fish will be ok in it or not. in some cases, or i might even say MANY cases, the LFS
will say whatever they can in order to make their rent.
if you were to purchase that fish at about 1.5"-2", you would have to find a home for it before it reached 4". these fish don't do well physically, in confined quarters.
once your tank is established, and all of your rock and livestock, and maybe corals are growing and they look good, it's a major traumatic situation for the inhabitants, and you, to have to catch and remove a fish.
you are better off finding a fish that is suitable for your tank.