Arrange the given words in the sequence in which they occur in the dictionary.
i. Rainbow
ii. Rancour
iii. Rattle
iv. Rainy

Let's sort the words step-by-step according to alphabetical ordering:
All words start with 'Ra'. Look at the third letter:

i. Rai... (3rd letter is 'i')

ii. Ran... (3rd letter is 'n')

iii. Rat... (3rd letter is 't')

iv. Rai... (3rd letter is 'i')

Between 'i', 'n', and 't', 'i' comes first. So words (i) and (iv) will come before (ii) and (iii).

Compare (i) Rainbow and (iv) Rainy up to the 5th letter:

i. Rainb... ('b' comes first)

iv. Rainy... ('y' comes later)

So, sequence starts: i, iv

Now compare (ii) Rancour and (iii) Rattle by their third letters:

'n' comes before 't'.

So, (ii) comes before (iii).

Combining everything, the complete dictionary order is:
i, iv, ii, iii (Rainbow, Rainy, Rancour, Rattle)