Only One Side Of My Controller Vibrates In The Rumble Test, Is One Motor Dead?

What you describe is very likely the design rather than a fault. Like most dual-rumble controllers, the DualShock 4 has two motors with different jobs. The left one spins a heavier weight and produces the deep, strong rumble. The right one spins a lighter weight and produces a higher, buzzier vibration. With both at full power, the left dominates what you feel and the right can seem weak.

Browsers drive these two separately. The vibration test sends a strong magnitude to the heavy motor and a weak magnitude to the light one, which is how the Gamepad API describes them. So the useful check is to run each one alone.

Set strong to zero and weak to full. A healthy right motor gives a clear, fast buzz you can feel through the right grip. Then swap: strong full, weak zero. The left should give a deep shake.

Signs of a genuinely failing motor look different: no movement at all, a grinding or rattling noise, a motor that only starts after a tap, or one that cuts out after a second or two. Those point to a worn motor, a loose weight or a broken solder joint.

At four years old a tired right motor is possible, so the isolated test is worth doing before ordering parts. If it does fail, the motors can be replaced, though on a DualShock 4 they are soldered to the board.