No ... you don't. I saw a headline this morning on Yahoo warning owners of 4 and previous versions NOT to upgrade to iOS8.
Thank you for the advice, I won’t upgrade, and I’m happy about it.
I’ll get a new phone in 6 months (March) when my contract runs out, and use my 4 as a iPod.
A well-intentioned but pretty much pointless warning. iOS8 won't run on anything older than an iPhone 4S. It's running great on my 5S, but I'd hold off on the 4S. If past experience is a guide, it will be slow on the oldest supported machine out of the box, but a .1 upgrade in the near future will improve that.