create schema with optional relations
Rule of thumb: fewer tables and more columns.
In your case you should probably have one tier table, and make the level another column. Then have a constraint that level can only be between 1 and 4.
If the tiers are always in order and have those values you don’t need any more data — the absence of a record with level 3 would imply tiers 1 -&gt; 2 -&gt; 4.
You might need two columns for keys: one ID for each individual record and a second column which has the same value across each group of tiers that belong together. It’s that column that you would refer to in a foreign key from (an)other table(s).
But without knowing more about your exact use-case I can only keep this advice generic.