You can set the number of results per page in Google Custom Search admin site: https://www.google.com/cse
Please login, then chose your search engine, then go to Search features menu. Chose Advanced tab, open Websearch Restricts section and enter the results per page number into Result Size field. Default value is 10, if you leave empty.
I suppose you will find here 4, as I see 4 results per page on your site now.
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