SIMid and others, rear quarter panel on many aussie built cars is not removable, an ED is one of those...
To replace that rear quarter will involve very specialised cutting, welding, filling and smoothing... it's certainly beyond panel beating.
Also, the boot lip spoilers, if being priced as genuine (being insured by shannons, this could be the case) are very expensive, they can generally go for $150 in shithouse condition second hand, new would be more. Also there is the rear garnish which IIRC is slightly different (Different coloured stripe each side of the ford badge?) which are NLA so would be hard to find (or again, expensive if you find them) Then of course there's paint... and the spoiler is rubber so different than normal to paint, plus blending the C pillar, and driver side rear door, paint rear quarter, bumper, boot lid and probably blend left hand side also i'd imagine...
I can see how that adds up to $4000+ easily, the majority of the costs are in the quarter panel. I've cut one of these cars up with a gas axe to make it easier to move and I cut thru just under the C pillar, there are about 5 different layers of steel to do with the car's structure under there and by the looks of the fold just below the c-pillar cover on his car, some of that is probably also bent.