I have an old Galaxy S phone running Android 4.0 and only want to use the phone for calls. Does the OS version matter?
If you use it only to make and receive phone calls, then no, the OS version doesn't really matter. You might still want to keep up with updates, the latest official version of the OS usually bring improvements to battery life, bug fixes, and other little improvements; but keeping up with updates is much less important if the phone is just used as a basic phone. The protocols serving basic phone functionality (GSM, SMS, MMS, etc) is pretty well standardized and the carrier had all kind of incentives to maintain backward compatibilities.
You might want to turn off mobile data if you're not using it.
If you are using the phone's browser regularly and/or running apps, then is crucial to get the latest version of the OS and the browser. The latest version of the OS and apps also provides security fixes, so running an outdated OS or app can be a risk to you, your data, and to your other devices.
If you are running social apps (e.g. chat, social media) or other apps that relies on an external service (e.g. news feed, sync folder) then it is pretty crucial to keep the apps up to date. Some apps are better than others, but these apps, as they rely on external services, may regularly break functionalities for people that doesn't keep up with updates, as the service evolves. For these apps, it's easier if you keep up to date than to wait until things start breaking before updating.
For purely offline apps, like many stand alone games, keeping up to date is less crucial. You may choose not to update them as long as the current functionality works for you.