Being a non native speaker of English, this type of sentence always bites me:
It was a pleasant four hours journey from Nashik to Pune.
Should it be hour or hours?
As kiamlaluno comments, the standard form is a two-hour wait, three-man band, four-course meal...
Although not everyone always bothers with the hyphen, you should include it if you want your written form to be above criticism. But never pluralise the "unit" (two-hour, three-day, four-month, six-foot, etc.).
In this sentence, the phrase four hour (sometimes written as four-hour) acts as an adjective, and English adjectives do not have a plural form.
This type of adjective is called a compound adjective. They are commonly used when describing time and size (among other things).
Some other examples include:
I've seen this written – in published works – at least four different ways:
That all said, when looking for examples, it was relatively easy to find examples of formats (b) and (d), while examples of (a) and (c) were much harder to find, and seemed to come from less reliable sources. I would recommend using either (b) or (d) in writing.
It's hard to gauge which forms are preferred, because Ngrams aren't much use here – there's just no way to check all possible usages (e.g., we could check two hour trek, three hour tour, four mile hike, five day sojourn, six hour walk, seven day trip, etc.).
In case anyone thinks (a) and (c) are not valid, don't argue with me, argue with the publishers of these books:
It was a pleasant four hour journey from Nashik to Pune.
...is correct. "Four hours" would definitely be wrong here. But bear in mind you may also see:
It was four hours' pleasant journey from Nashik to Pune.
The difference is the "a" - in the first example, you're talking about "a journey". In the second, the focus is on "four hours" and how you spent them. Both are correct, and idiomatic, but do not mix them.
If you wish to use the second, don't forget the apostrophe. :)