We have started a new site for healthcare. In that there are 4 main pages regarding health we have worked SEO for 3 pages but we got page rank for 4th page which we have done nothing for that . On what base Google has given page rank for that?
On the basis of your over all site Back links plus also the information or Content you shared is fresh and informative for others this is might be the reason that you are ranking in Google, without much effort for that page.
If you got page rank for that page which you didn't do anything according to you,but I hope you will have quality and fresh content on that page, also perhaps you did good internal linking for that page,also may be you place any link on high PR website!
It is not essential to work for all pages for getting good page rank. The reason is that many of the seo experts use home page for getting good back links and use keywords related to the niches to get good keyword rankings. So as good number of backlinks will give your whole site a good rank.
This happened because you are working for other 3 pages and your 4th page is connected to them and according to PR algo PR is divided to all web pages.