Question on Iphone 4 Battery life and restoring

I've read that if I restore settings of Iphone 3G to Iphone 4 will affect my battery life and will need to do a factory reset in order to resolve the problem. Question is that if I do a factory reset do I need to delete my Iphone 4 backup on Itunes too? As my Itunes backup my Iphone 4 settings after I restore data to Iphone 4 using 3G data and will my battery still drain fast? Do I really need to start all over again to install all apps and contacts etc?