Using 4399 is very simple. The interface is divided into several different tabs from which you can easily access entries for loads of apps and games. You can see lists of what's popular, take a look at specialized articles, and much more. You can even use the search box if you know what you're looking for.
To download any video game you don't even need to register. Just tap the download button for the game of interest, wait a few seconds, and install the APK. The game then starts. The app even tells you if there's an update for a game you've downloaded before.
4399 is a very interesting option to find Android video games, especially if you're interested in Asian titles. One of things to keep in mind, too, is that lots of these games come to China or Japan before making it to the West (although on Uptodown we usually have them much sooner).