Well, something is wrong....If this fish has been in copper for 4 weeks, there should be no more ich. After the first cycle the fish may appear 'clean' but the more it reproduces, the more ich in the tank, the more ich on your fish. You may never see it 'spot free'
Are you sure you kept the copper at the proper level? Did you test? If your copper levels were too low, the copper won't work
Are you positive it is ick? Could the Hippo be having a reaction to the medication?
4 weeks is enough, get the copper out..I would start dropping the SG slowly, maybe add an antibiotic such as Furan-2.
If you are sure its ich, I would treat with Hypo, do not use Copper on this Tang anymore