If you clear data, all your apps lose any saved data, so you don't want to do that. (If you're uninstalling an app, you'll regain the space it's taking for data.) To clear the app cache, go to Settings/Storage/Cache. But you won't clear much storage that way - most apps clear their cache when you close them.The phone probably has less than 4GB of free storage to begin with (the kernel, Android and system storage usually take up a bit more than 4GB), so it's a small phone for today's apps.