You are correct about the 4:3 ratio. The lowest common denominator was about the average household using a 800 x 600 monitor. In order to keep you best web work on the "top" of the page, one usually sets there area to reflect a 800x600 window.
Some programmers use a flexi window that adjusts to whatever resolution the monitor happens to be.
Changing your pics to a 4:3 aspect ratio involves cropping. You do not want to compress the image into that size unless you really want that result. I usually base the largest pixel number to 800 and then allow the other peramter to self adjust so that my photos stay in proportion.