Splitting a QSFP28 port on FortiGate 1800F/4200F/4400F into 4x25G with a breakout DAC

How do you break a 100G QSFP28 port on a FortiGate 1800F, 4200F or 4400F into four 25G subports with a QSFP28-to-4xSFP28 passive DAC, and why do the subports come up at the wrong speed?