tiers 1-3 are policies[0] for in-tree targets, so by saying tier 4 they mean one implemented in a fork. Though that kind of skips over targets that can get away with just a custom target spec[1] and not modifying the source.

[0] https://doc.rust-lang.org/beta/rustc/target-tier-policy.html [1] https://doc.rust-lang.org/rustc/targets/custom.html