Problem from D. Morin's "Introduction to Classical Mechanics"

I'm getting incorrect results solving Problem 3.30 Atwood's 4 from 3.7 Exercises of Morin's book, 2008 edition.

According to my solution, the lowest mass in the Fig.3.23 of the book for N=3 has an acceleration greater than g - acceleration of gravity, which is not possible for the problem's conditions. It looks like my 'conservation of string' equation - acceleration of the lowest mass is 4 times greater than acceleration of the top mass - is wrong.

Any help?