The required ampacity of service conductors is called for in 230.42(A), and is based on the load calculations (125% of continuous loads plus 100% of noncontinuous loads). The ampacity of the conductors is to be determined from 310.15, which includes 310.15(B)(2)(a). As you have stated: 6 current carrying conductors in one conduit needs to be derated to 80%.
230.42(B) also requires the minimum ampacity for ungrounded conductors to be not less than the rating of the service disconnecting means, which you have given as 400 amps.
Assuming 90 degree insulation, paralled 4/0 copper, or 300 aluminum would be the minimum, as 260 amps x 2 x 80% = 416 for copper, and 255 amps x 2 x 80% = 408 amps for Aluminum.
240.4(B) does not come into play on services.
