Choose one of the countries to be the home country. Set your foreigner % to 75%. Edit your nations.txt file so that all countries have a "0" for baseball talent, except the 4 countries you want players to originate from, make them a "5" (any non-zero number would work, but I would use 5). When the players are created, roughly 25% will be from each country.
The OOTP2006 customization guide (in the "mods" section) can give you more info:
http://www.ootpdevelopments.com/boar...d.php?t=122561