The phrase "fourth tier" refers to something that is at the lowest level or rank among a group of things. It signifies being in the last position or the least important. Full definition
For over twenty years, the coach had failed to win even a third or fourth tier championship in basketball. (kidsinthehouse.com)
Skills that heroes are naturally geared towards are easy to spot because those have an extra fourth tier available containing a single, powerful skill. (wolfsgamingblog.com)
The third and fourth tiers represent prevention and intervention, respectively. (pyramidmodel.org)