Well - I'm breathing a sigh of relief - and have learnt a valuable lesson; don't trust Amazon reviews. I can use the 4" hog wire...
The first ball I bought (
https://www.amazon.com/dp/B000KYQ406?tag=atomicindus08-20 ) is slightly undersized. Given that, and the fact it's made of rubber it only needed the slightest finger pressure to go through some of the holes in the 4" hog wire.
The second ball (which is wooden, and therefore better matches the code requirement for a rigid ball) is slightly larger, though I think (despite what the listing says), it's actually only 100mm (just under 4"). It doesn't go through the hog wire even if you press firmly. (
https://www.amazon.com/dp/B0DQ9HBTXJ?tag=atomicindus08-20 ). And as a bonus, my wife has decided she will use it for a craft project once I'm done
It is surprisingly difficult to measure a 4" ball with high accuracy with what I have available...but I've confirmed the rubber one is definitely smaller using my laser level to compare them, and the wooden one is definitely not larger than 4", which is what matters...
Thanks to everyone's input...if the hog wire wasn't going to work I'd pretty much decided to use horse wire (with 2" x 4" holes) - thanks to @
dcg9381 for suggesting that....