Increase Diameter on Pump Suction Only?

NFPA 20 specifies a minimum size for suction pipe before a fire pump, to reduce the water velocity and avoid cavitation (among other things). NFPA 20 states that this only applies to 10-pipe diameters of length before the fire pump.

We have a very long run from the backflow preventer at the service entry and the fire pump room. Hydraulically, this long run works with a long 4-inch feed, which would then upsize to 6-inch right before the 450 gpm pump for the 10-pipe diameters.

My question is, am I allowed to drop the main size between the backflow and the fire pump? 

Hydraulically I don't see why not, we're not dipping below 20 psi anywhere on the system and we appear to meet code. Intuitively I've never dropped pipe size to pick it back up, but this situation with the pump feels a little different. This is above a multi-family unit, so 6-inch steel is nearly impossible to fit within short open-web joists and would be a major issue for the building owner if we had to drop a soffit.

​Thanks in advance for your take!

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