2000 nissan patrol engine failure HELP

Hi our 2000 3.0L patrol began running rough and became difficult to start, off to the mechanics we went. Number 4 full of coolant (familiar story so far). Removal of cylinder head showed cracks in pistons 1,2&4. cracked at cylinder 1,3&4. We have had the car regularly serviced and it has only done 150000km. Our local dealer put a claim in to Nissan HQ, There response was to offer to pay 50% of a bottom end rebuild kit parts only,= about $900.00. We have been quoted $6000.00 for repairs. When contacting Nissan HQ lets say they are being less than helpful even though one customer service rep I spoke to admitted a fault with the engine but had a too bad attitude. Any help or suggestions would be appreciated