Thanks for the correction. My working sketch now has:
// Mega series w/'incompatible' ethernet shield needs this + jumpers
// MOSI = D11 -> D51 MISO = D12 -> D50 SCK = D13 -> D52
pinMode(4,OUTPUT);
digitalWrite(4,HIGH);
pinMode(10,OUTPUT);
digitalWrite(10,LOW);
pinMode(11,INPUT);
pinMode(12,INPUT);
pinMode(13,INPUT);
pinMode(50,INPUT);
pinMode(51,OUTPUT);
pinMode(52,OUTPUT);