Is it possible to create a 4 inch by 4 inch channel through concrete using acid?
Offhand, this seems dangerous to the structural integrity of the slab, because the acid would follow the pore structure of the concrete and weaken the wrong places.

However, concrete cutting saw rental is available; e.g. United Rentals
and http://www6.homedepot.com/tool-truck-rental/Walk_Behind_Concrete_Saw_14/MK-CX3/index.html. Two 45-degree cuts ~6" or deeper should do the job.