This is a debatable issue. I would start by asking the reason (basis) for needing a 4000 amp service. If an Article 220 load calculation was performed, and if that calculation showed the load to be (let us say) close to, but not above 4000 amps, then it may be possible to justify using the 420 amp rating that appears in Table 310.16. The reason is that the NEC load calculation process is very conservative, and the actual load is likely to be well under 4000 amps.
On the other hand, if you intend to load this facility to every bit of that 4000 amps (which you can do, if not all of it is continuous), then I believe the requirements stated at the top of Table 310.16 are not met (i.e., not more than three current carrying conductors). I am not talking about derating for more than three conductors in the same conduit. I am talking about derating for more than three conductors in close proximity. Heat generated in one conduit will be transferred to the surrounding dirt. The ability of any conductor to reject its heat to the surrounding dirt will be adversely impacted by the fact that there are other conductors, in other conduits, adding heat to their surroundings.
I strongly suspect that the engineer performed a formal calculation of the ampacity of each conductor in this ductbank, and that the 298 amps is the result of the calculation. I have performed such calculations myself. I have never seen a result that was at or above the value in Table 310.16. So my answer to your ?is this correct?? is that I think it probably is correct.
