Blend4Web is described as 'tool for interactive 3D visualization on the Internet' and is an app in the Photos & Graphics category. There are more than 10 alternatives to Blend4Web for a variety of platforms, including Windows, Mac, Linux, Online / Web-based and iPhone. The best alternative is Godot Engine, which is both free and Open Source. Other great apps like Blend4Web are Unity (Freemium), Unreal Engine (Freemium), Verge3D (Freemium) and Sketchfab (Freemium).
A game-engine fully integrated in Blender UPBGE is an open-source, 3D game engine forked from the old Blender Game Engine and deployed with Blender itself. This unified workflow is its main strength, as you can make your game from start to finish without leaving Blender.