You could make it work for 1600 amps with 8 350 kcmil in each raceway, assuming that the grounded conductor is not required to be counted as a current carrying conductor. The 90° ampacity of 350 is 350 amps and we can use that for purpose of derating. The derated ampacity would be 280 amps and 6 conductors at 280 gives you a total ampacity of 1680 amps. Ten 350 kcmil THWN-2 will fit in the 4" rigid so you will have room for the EGC. Note that it will have to be a 4/0 for each raceway.
Don
