Don't redesign the wheel FRM, I'm sure the builder knows what they're doing. Series wiring (assuming they aren't already) changes other parameters.
Even though they don't publish an impedance curve that I see, the builder said the 4ohm version doesn't dip below 4ohms.
They simply made available a different version for folks who have tube amplifiers that don't have a 4ohm tap or have high fixed output impedance typical of SET.
Apparently, that version is a slight compromise in performance. No need for the OP to compromise. The builder recommends they buy the 4ohm version.