Well, getting hyperthreading going on a stock kernel seems to be a bit tricky, but here are the options I used to get it going.
I think the most important of those is
CONFIG_NR_CPUS=32 as it wouldn’t find more than 2 processors with it set to 4.
dmesg showed 4 cpus, but 2 of them were number CPU_6 and CPU_7.
Oh well, I see 4 little penguins on boot, so it’s working now.