OK - first off the 4.5 oz is way too much to add to a tail. [X(]

Next, I would suggest that you have a second opinion / inspector to have a look at the AC and confirm your point of balance. (just in case)

IF the balance is confirmed start moving things...
-Move the engine as close to the firewall as possible,
-Change the Engine mount to a lighter style,
-Get a light spinner,
-Shorten the control rods and move the servos as far aft as possible,
-Convert the AC to a tail tragger.

Do anything and EVERYTHING before you add weight to the tail.

With ARFs the quality of the wood is occasionally questionable, and it is possible that you have the nose made of a 'Heavy' type of wood - this can affect the balance...[]

Good Luck!