Unfortunately, in 4B engines the pre-combustion chambers were not sold as separate parts, so there is no part number and you technically would need to buy a new cylinder head, which will almost certainly no longer be available.
The 3BII uses very similar engine architecture so you could try to use those, but no guarantees. They are 11106-58020 and appear to still be available.
It happens that I have a 3BII pre combustion chamber here (used/damaged) so I can measure it if you want to compare. I've never seen a 4B engine and would like to learn more.
EDIT: I see RADD Cruisers sells aftermarket 3B pre-combustion chambers and has precise dimensions listed on their site:
3B Pre-Combustion Chamber - Toyota 3B (pre-cup) 11106-58020 - https://www.shopraddcruisers.ca/3b-pre-combustion-chamber-11106-58020.html
It also links to an Engines Australia document on cylinder head damage underlying the 'dropping' of a pre-cup and the very precise fit/protrusion required. This is most likely why Toyota decided not to sell them separately.