They aren't tier 4... They are tier 3.
Look the mercs are a bit more complicate at level tier relation.
Here is how merc's tier and level works.
A merc receive xp after every fight (same as hero) also they have their own xp requirement to advance to the next level. (you can see current and next level xp by leaving the cursor upon the little yellow shield with the number of their level). This means that if you hire a merc level x9 he can advance to (x+1)0 level after some battles. But this will not advance him tier, he will simply level up according to the level up rules for mercs(
http://forum.imperialhero.org/int/index.php?topic=3375.0 ).To change your merc tier, they must have reach the necessary level for the new tier, and visit a healer's shop with tier higher than your current merc's tier and upgrade him for some gold. Note that you are able to hire or upgrade your mercs at tier more advanced than yours but you will not be able to use them in battle.