Our DD loves the autographs, I agree with SammieG, it gives them a good reason for interaction and something to ask the characters for.
You don't even have to have a Disney one if you don't want to. We got a neat one that was only $12 but it had a pen on it and a photo holder for each page also. It is really cute.
If you don't do it you 4 year old will be fine. However if you want to it can't hurt. DD loves looking at them. One year I cut them up and put them in the photo album scrap book, the rest of the years I just got a Disney book. One book lasted us a few trips and when we ran out of room we just got a new one. I like the ones where the pen stays with it since I am forever searching for the pen in my bag and miss photo opps because of that.
Have fun!