LEVEL FOUR IS A SPEND AND A REPUTATION WALL
Game mode
Region
Flea market
How it is delivered
A trader at level one sells starter kit. Level four is where the ammunition, the armour and the weapon parts people actually want appear. Getting there needs a spend threshold, a reputation threshold and usually a quest chain, which is three different kinds of work rather than one.
He is the one trader who is not available from the start: a quest unlocks him, and that quest has its own requirements. The Jaeger row covers the unlock and the loyalty levels together, because selling the second without the first would be selling something unusable.