Arrange the given words in the sequence in which they occur in the dictionary.
i. Treadmill
ii. Treason
iii. Treacherous
iv. Tread

Let's sort the words letter by letter as they appear in a standard dictionary:
All words share the prefix "Tre-". Let's look at the 4th letter:

i. Treadmill (4th letter 'a')

ii. Treason (4th letter 'a')

iii. Treacherous (4th letter 'a')

iv. Tread (4th letter 'a')

Since all have 'a' as the 4th letter, let's look at the 5th letter:

i. Treadmill (5th letter 'd')

ii. Treason (5th letter 's')

iii. Treacherous (5th letter 'c')

iv. Tread (5th letter 'd')

'c' comes first, so iii. Treacherous is 1st.
Next is 'd', shared by i and iv. Let's look at their lengths and subsequent characters:

iv. Tread (ends here)

i. Treadmill (continues after "tread")
Hence, iv. Tread comes 2nd, and i. Treadmill comes 3rd.
Finally, ii. Treason (with 5th letter 's') comes 4th.

The complete dictionary sequence is: iii, iv, i, ii.