not sure about the different shapes but any of the fly boards have the same shape but they are much lighter and have a different core i believe. Ive heard the s4 is a ps3 on steroids, and all I know about the Ennen is that its a flex board
if you dont ride it to much it wont make much diffrence if you have rid it loads and is maybe sun damaged as well then it might be worth getting a new one for the extra pop but if you want to save money get a 06/07/08 ps3 there all the same appart from the binding inserts and the graphics the newest soven pro model is diffrent though! maybe try and get a ride on a 08 ps3 with a grind base see if you like it. (they last a bit longer but dont ride quite as sharp)
and those lighter boards liquid force are making.......let a few other people with big wallets test them first to see if they last and dont break to easy.