A 3 ohm minimum impedance is pretty typical for a 4 ohm speaker.
Using a 4 ohm speaker on the 8 ohm tap of the 8K:8 ohm transformer will present a load of probably 4200-4300 ohms to the tube. (It's more than 4K because the copper losses are proportionately larger in this case). That's not enough different from the 5K you would get from an EXO-44 wound for 4 ohms, to be worth it.
I think it's quite reasonable to try the amp as is. The damping factor will be kind of low, which might make the bass a bit wooly, but then you're not going to get astounding bass from an 8" speaker anyhow. :^) Triodes are pretty tolerant of impedance mismatches.
It's overkill IMHO to replace the transformer with one that has a 4-ohm winding, but you could in theory ask Mike if he would make a custom version of the BH-1 with a bifilar secondary, giving taps at 2 and 4 ohms without changing the amount or size of copper on the winding.
When we were developing the new Stereomour and SEX output transformer, we had several prototypes made. I'll look through the miscellaneous-iron box and see if there's something suitable, if you are interested.
