articles - Why 'an' is needed before the word 'extra'?

She must now sell two properties she owns or face an extra four years in jail.

As a rule of thumb, fours years do not go with "a" or "an". Why, in this case abover, 'an' is a must? Look at the following example:

1. The unions says it was expecting at least an extra 800,000 journeys during the Games.


2. The unions says it was expecting at least extra 800,000 journeys during the Games.

Usually, an or a is definitely not required before plural noun. But it seems I should choose Option (1) here. Why?

Anther example:

As a result of the expansion of the manufacturing plant, production capacity will increase by about 25 percent, and a further 85 permanent full-time jobs will be created in Huntington.

Answer

From snailboat's comment...

the exceptional use of the indefinite article a(n) when the head noun is plural and has an attributive modifier

That all sounds rather complicated to me, and I'm a native. However, the need for a(n) is because of the word 'extra', rather than the number following it.

Consider this:

She must sell the houses or face four years in jail.

Perfectly correct, the number alone does not take an article.

She must sell the houses or face an extra four years in jail.

Now we must agree with extra (this is our 'attributive modifier'), which does take an article.

All of these do

an extra four years
an additional four years
a further four years
a minimum of four years

a maximum of four years

& probably many others.

They are all modifying our initial 'four years'.