Fourth position
4th position sits between 3rd and 5th the same way 2nd sits between 1st and 3rd — no landmark of its own, found by ear or by shifting partway between two positions you already trust.
The note map
Why it's last in the teaching order
4th position is genuinely useful — but like 2nd, it's usually substitutable. A passage that
seems to want 4th position can often be played just as well by staying in 3rd and using an
extension (reaching-one-more-note), or by shifting straight to 5th a little early. That's why
this course places it after 5th rather than before: by the time you meet it, you already have
two solid neighbours to find it between.
Try it
Try
Shift from 3rd position toward 5th on the A string, but stop partway — check your 1st finger against E (4th position). Then try arriving at the same note by shifting down from 5th instead. Both directions should land on the same pitch; if they don't, one of your two landmark positions has drifted.
The usual mistake
Treating a landmark-free position as "less real" than 3rd or 5th and rushing past it. 2nd and 4th ask more of your ear precisely because they have no physical shortcut — that's more practice value, not less, even though they show up in less repertoire.