Not quite.
The 4-track section ended (going west) just before the New Haven crossing in Framingham (where today the branch from Mansfield connects), and became two main tracks crossing Concord St. Track 4 then started again just past the Framingham station, and continued out to Ashland, where it ended just before reaching the station there.
The "third iron" and "fourth iron" names actually have nothing to do with track numbers on the 4-track section. Those were 1-2-3-4 from north to south.
A better map to use would be at
http://zekedev.com/sites/boston_line/index.cfm which is a complete track chart of the line (showing all of the side tracks as well as station locations) from 1950.