With the new phone not being as rounded as the 3g you may struggle to get it to fit. You could always go for the universal one as I did and then just buy the inserts everytime you get a new phone if needed. Come to think of it, the ip4 may fit in a 120gb ipod insert anyway, perhaps even with a thin case on it.
As each generation has been released there have been a lot of issues with old power supplies, ie wall chargers/car chargers and battery packs not being compatible with the new model, giving the ' this accessory is not supported ' meassage onscreen. We won't know until its out. I hope they haven't changed anything this time, last year I had to sell my battery pack and get a new one.