I can't find anything to clarify this on the government website. Still just has the old tier 1/2 rules. Got this from the Beeb:
"Up to 10,000 fans or 25% capacity, whichever figure is lower, will be allowed at larger outdoor venues, while at smaller outdoor venues, the limit will be capped at 4,000 or 50% capacity."
Logically that would put the threshold for what constitutes a larger/smaller venue at 16K, meaning that anything from 8K to 16K capacity can have 4K spectators, then up to 40K @ 25% and beyond that 10K maximum. Meaning we'd be good for just short of 8.2K.