It is an older one, I've just heard so many times that 4000s size up massive (as big as 28 in some cases). I'd like to ride the biggest tyres I can, and honestly don't expect to fit 28s, but was just curious...
Then stick with 25mm (actual), as there's a chance your chainstay will get worn through due to tyres rubs and result in needing repair/new frames if using larger tyres.
It is an older one, I've just heard so many times that 4000s size up massive (as big as 28 in some cases). I'd like to ride the biggest tyres I can, and honestly don't expect to fit 28s, but was just curious...