the LFG drivers do not work well on older machines - i tested them all on all machines last year.
and it is a hardware limit - on bus bandwidth - doing 4 inputs saturates the pci bus bandwidth - and doesnt leave anything left to do anything else-
i do not recommend anyone uses LFG4 on anything less PCI-e machines.
It is too much for older g5's with their limited buses -
there were some later generation dual 2.0 G5's which were also crippled by apple on the pci bus.
but you can put 3 LFG4 cards - for 12 inputs into an intel mac pro - and it does work.