Capt Coco
Bottom line to you point is 'the costs'. Quite right. 777-300 is extremerly efficent, however, the 748 is the first 4 engine aircraft to have lower fuel costs per Pax/Kg of cargo than the 777-300! 748 for Japanese airlines anyone?
While I would agree the market is rather small for the 748 (and 380) it has been less expensive to develop than the 380 and will probably take enough orders to cause part of the 380 market to leak away.
I'll be interested to fly both with LH , they have ordered both, as it will hopefully give a good comparision as the Airline product should be the same (apples with apples) and just the airframe different.