Wow, that sounds incredibly frustrating and counter-intuitive to the goal of having a high-tier monument! You're completely right—if a Tier 4 monument can be pushed back by a new, lower-tier claim, there's absolutely no incentive to spend the massive resources required to upgrade past Tier 2 or 3. That essentially breaks the late-game territorial mechanic.
Both of your options seem like logical fixes to restore the point of high-tier investment:
sprunki phase 5
Option 2 (higher tier can't be pushed by lower/equal) is probably the cleaner, more direct solution for establishing a stable territory hierarchy.
That definitely needs to be addressed for green worlds to make any sense long-term.