With every update Apple does not only release new features, but also fixes bugs found previously, and adds to the security of your device. It would be a good idea to keep your iPhone up to date, imo.
Depending on the software on your device (especially with iOS 7 or less), it will also become more and more difficult to find apps which work with that iOS, if you don't update.
With an 8GB iPhone, I'd recommend updating via iTunes. This will install a fresh version of the software on your device, which means it's not very likely that you'll experience issues after updating. Before updating, backup your iPhone.
As soon as you've finished updating, disconnect your device from iTunes, and reset it: press and hold Home and Power button simultaneously until you see the Apple logo. As soon as it appears, let go of both. Your iPhone will power up on it's own. This reset won't remove data or change settings. It might help preventing bugs you'd experience otherwise.
On the other hand: the iPhone 4s was released in 2011. Hardware has improved enormously since then. Although the latest software is intended to be used on your device as well, your iPhone might become a bit slower, e.g.