EQUI_HotButtonWnd is the file you're after
Study the code for HB 1&2, specifically the page left/page right and page label - copy these to HB 3&4 along with the corresponding x/y for each button
Make sure you adjust the size of HB 3&4 to 1&2 as well (in the area where it lists all the <pieces> for HB 3&4)