If you want to add more portlets to a page or change default layout of the page by going to Manage Pages Portlet, selecting the page that you want to Edit and clicking on its Edit Page Layout button like this
It will open Edit Layout page like this. On this page Portal provides 6 different options to choose from, that you can choose to define what layout of the page you want
If you have advanced requirements that cannot be fulfilled by one of the default layouts then click on Show Layout Tools Link, if you dont get that link Go to Configure mod of the Edit Layout portlet and check 'Show toggle link for "Show layout tools/hide layout tools"' check box
Now when you come back to the Edit Layout page, you should get this link, click on the Show Layout Tools button and you will get a more flexible layout like this, now you can click on the add column container or add row container buttons to create a page with say 4 columns.