Yes, as you worked out, that says 4.5 Powers special chip. If you want a definitive as to what's in there, ask Dom at TVR Power. Might involve sending it to him.
Does your car run fine on it? All the ECUs are the same except for the chip.
Not utterly sure that a 4.0 and a 4.5 are that different - similar heads etc. and anyway, someone may have changed the chip without removing the label.