Android - Disable lock-screen enter/return button?

No, because unlike the iphone/ipod touch, the unlock PIN can be longer then 4 digits. Android allows between 4 and 16 digits for the lock screen. Since it is "unknown" what the length of your pin is, you have to press OK to indicate "you are done".

Some of the alternate lock types unlock immediately.

The built-in Android pattern lock, where you do a "join the dots" between the nine dots displayed on the screen unlocks immediately your finger lifts off the screen (presuming that you've got the pattern right).

Yes, the question is old but remains valid for current time as well, so I'm answering keeping currently relevant Android versions in mind.

If your Android is rooted, has a compatible Xposed Framework¹ installed, then provided that your Android can run GravityBox Xposed module, the latter can solve the problem.

Instructions:

1. Install GravityBox relevant to your Android version, enable the module under Xposed Installer app and reboot the device.
2. Launch GravityBox, go to Lockscreen tweaks and enable Quick unlock. Once you enable it, the previously grayed out entry named PIN length would become available for tweak. Set it to the length your current PIN/Password consists of.

Changes would take place immediately.

_{(Click image to enlarge)}

Official GravityBox versions:

• For Android Jelly Bean: GravityBox [JB]
• For Android KitKat: GravityBox [KK]
• For Android Lollipop: GravityBox [LP]
• For Android Marshmallow: GravityBox [MM]

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¹ _{Xposed Framework: Android 4.x, Android 5.x and 6.x}