The higher the credit per day, the more senior it goes generally. 4 days can be held by junior pilots because #1 the vast majority of trips of built to be 4 days. 3 days are pretty common as well. In my experience locals and two days, particularly ones that pay well, go to senior in domicile pilots. (opposite of 4 days, there aren't very many of them) Exception to the rule might be SkyWest ORD where a large percentage of pilots commute. But even then, they do go kinda senior. My experience to say this comes from SkyWest and Trans States. Other airlines may be different.