With Sony's PS4 DualShock 4 controller now officially supported in Valve Corporation's digital distribution platform known as Steam, here's a guide on how to use a PlayStation 4 DS4 controller as a PC gamepad on Steam! 
To quote from Tom's Guide on How to Use a PS4 Controller on Steam:
"To get your DualShock 4 set up on Steam, launch the Steam client and click the controller icon at the top right to enter Big Picture Mode, which is Steam's console-like, controller-friendly interface. From there, go to Settings > Controller Settings and make sure "PS4 Configuration Support" is toggled on.
After that, simply plug your PS4 controller in via a microUSB cable or Sony's DualShock wireless dongle. You'll be prompted with a small window for personalizing your controller, which lets you assign a name to your DualShock 4, toggle the rumble feature and set the color, brightness and saturation of the gamepad's top-facing lightbar.
Your DualShock 4 will now be registered to work on Steam, but that's just the beginning of the fun.
If you head to Settings > Base Configurations, you can create button schemes for navigating Steam in both Desktop and Big Picture mode, as well as dictate what happens when you hold down the home button in-game. You can also create game-specific control schemes by simply launching a game, pressing the PlayStation button and selecting Controller Settings.
The amount of ways you can customize your DualShock 4 on Steam is staggering, and reminds me a lot of the fine-tuning options that Valve offers for its own, less-conventional Steam Controller. In addition to remapping every button and adjusting the sensitivity of the thumbsticks, you can also take advantage of the controller's unique touchpad and gyroscope in a myriad of ways.
Want the DualShock 4's touchpad to behave like a mouse? Go for it. How about controlling the in-game camera by simply moving the controller? You can make that happen. If you like tweaking every possible parameter when setting up your controls for games, Steam's DualShock 4 features could keep you busy for hours."
From Steam's Latest Update, to quote in part: Controller
To quote from Tom's Guide on How to Use a PS4 Controller on Steam:
"To get your DualShock 4 set up on Steam, launch the Steam client and click the controller icon at the top right to enter Big Picture Mode, which is Steam's console-like, controller-friendly interface. From there, go to Settings > Controller Settings and make sure "PS4 Configuration Support" is toggled on.
After that, simply plug your PS4 controller in via a microUSB cable or Sony's DualShock wireless dongle. You'll be prompted with a small window for personalizing your controller, which lets you assign a name to your DualShock 4, toggle the rumble feature and set the color, brightness and saturation of the gamepad's top-facing lightbar.
Your DualShock 4 will now be registered to work on Steam, but that's just the beginning of the fun.
If you head to Settings > Base Configurations, you can create button schemes for navigating Steam in both Desktop and Big Picture mode, as well as dictate what happens when you hold down the home button in-game. You can also create game-specific control schemes by simply launching a game, pressing the PlayStation button and selecting Controller Settings.
The amount of ways you can customize your DualShock 4 on Steam is staggering, and reminds me a lot of the fine-tuning options that Valve offers for its own, less-conventional Steam Controller. In addition to remapping every button and adjusting the sensitivity of the thumbsticks, you can also take advantage of the controller's unique touchpad and gyroscope in a myriad of ways.
Want the DualShock 4's touchpad to behave like a mouse? Go for it. How about controlling the in-game camera by simply moving the controller? You can make that happen. If you like tweaking every possible parameter when setting up your controls for games, Steam's DualShock 4 features could keep you busy for hours."
From Steam's Latest Update, to quote in part: Controller
- Added Steam Configurator support for PS4 Dual Shock Controller. Enable in Big Picture settings Add/Test Controller settings. When enabled, PS4 controllers will have access to the same sort of customization/configuration support as Steam Controllers, including native API support. PS4 Controllers using this system can map the trackpad, gyro, buttons, etc. to keyboard, mouse, or x-input outputs and can make use of action sets, touch menus, radial menus, and so forth.
- Added software gyro+accelerometer integration for IMUs without hardware integration. Allows Joystick-Move/D-Pad/Mouse Leaning support for the PS4 gyro input.
- Added New Mode 'Joystick Mouse'. This is a joystick based mouse control useful for stick based controllers.
- Added controller preference setting for haptics. Haptics can now be globally disabled for a controller or default to the configuration preferences.
- Added support for LED Color setting. Controllers which are capable of setting an LED color can now set this in controller personalization.
- Added LED Brightness/Color controller action binding. The light on the controller can now be changed via a binding. This is useful to mark changes in state such as different action sets or mode shifts, or on start/release press activators for changing while a button is pressed, for example. Light settings are reset to user preference when changing applications/configurations.
- Added software calibration for non-Steam controller based IMUs to counter for gyro drift.
- Modes and bindings which are unavailable to desktop and Big Picture modes will no longer be shown in the configurator.
- Steam now shows a dialog when trying to edit a config for a controller which has opted out of being configurable.
- Unified Radial Menu Button Press/Release behavior on digital inputs such as physical face buttons or d-pads.
- Dismiss quit menu when minimizing or turning off a controller.
- Controllers now support a default Rumble Preference to be On or Off. Games can override this setting or use the default preference for that controller. Setting is available under each controllers Preferences in Controller Settings in Big Picture.
- Individual games can opt out of using Steam Configuration support for the PS4 controller under each game's Controller Settings. If a game has native PS4 support this allows it to continue to be recognized directly.
- Changed mode shift button activators to be uninterruptible by default.
- Changed Mouse mode Gyro Enable Button to default to Always On when placed on a PS4 Controller.
- Changed Joystick Mouse to default to Wide Response.
- Configuration Browser can now be toggled to show all available controller types rather than the default of the in-use controller type.
- Configurations can now be marked as 'Unique' to a specific controller. This configuration will only be applied to that controller for that user's account. By default, all controllers of a specific type will share a configuration unless marked as Unique.
- Fixed a bug where rumble would send to the incorrect controller and could cause controllers to control the incorrect player.
- Fixed default settings not applying properly to mode shifted modes, which could cause certain parameters to be incorrect by default on mode-shifts.
- SteamOS users should update to the latest version of SteamOS, as described here.
- Linux users should consider upgrading your udev rules to allow Steam access to /dev/hidraw* , as described here.
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