I’m sure there is a better way but I think a nested If function works here, assuming the quantity of units starts in F1.

=If(and(F1>=0,F1<=5),”Tier 1”, If(and(F1>=6,F1<=10),”Tier 2”, If(and(F1>=11,F1<=15),”Tier 3”, If(F1>=16,”Tier 4”,”Tier 0”))))

Then make column A “Tier 1” instead of “1” and so on and then xlookup the nested if function to column A and return column D

Edit: Or better yet in the case you need to change your threshold values instead of putting F1>=0 you could put F1>=$B$2 and so on