[Grammar] It's "four hours' walk" or "a four hours' walk" ?

[on the way]

My question goes as stated in the title above. It really baffles me whether an indefinite article should be used in such cases. I will appreciate it very much if anyone could tell me which of the following are correct.

a. It's four hours' walk.
b. It's a four hours' walk.
c. It's four-hour walk.
d. It's a four-hour walk.

What if the "walk" is replaced by "drive"? Does the rule, if there is one, still apply here? Any reply is much appreciated.

 

[probus]

Walk or drive makes no difference.

You omitted the most natural one "It's a four hour walk."

But there seems to be an epidemic of hyphen-insertion going on around here.

 

[Barb_D]

A is acceptable.
C is what I call correct.

I don't usually disagree with Probus but I must here. All the style guides I know say the hyphen is required.
A ten-foot ladder.
A two-hour drive.
A four-mile hike.
And so on.