There is a procedure in the maintenance manual to transfer fluid between no1 and 4 hyd systems , yes it involves a bit of switching, park brake applications etc etc but it is there and the fluid transfer does happen.
It seems to happen more on the 400 than the classic but that's probably because EICAS is easier to see and read the guages.
Or that brake source selection is automatic.
quite often you will find no1 hyd at 110% + and no4 sys at 80% a few applications of the park brake and no1 & 4 ADP and it's back to 100/100.