The correct answer is 4. Its ability to be turned off by a gate signal.

Gate Turn-Off thyristors (GTOs) are high-power semiconductor devices primarily used in applications requiring high-power switching. Unlike other thyristors, a key distinguishing feature of a GTO is its capability to be turned off by a negative gate signal. This characteristic is paramount for its use in high-power switching applications because it provides a controlled and efficient means to stop the current flow. Other characteristics, such as switching speed and thermal stability, are important for performance, but the ability to actively turn it off is essential for controlled switching. Without this gate turn-off capability, controlling the high currents involved in high-power applications would be significantly more challenging and potentially less safe.

Let's examine why the other options are less crucial:

• Option 1: High-power switching inherently deals with high currents, not low currents.
• Option 2: While high thermal stability is desirable, it's secondary to the ability to control the switching action itself.
• Option 3: Fast switching speed is important for efficiency but is secondary to the fundamental ability to turn the device off reliably.

Therefore, the ability to be turned off by a gate signal is the most critical characteristic of a GTO for its use in high-power switching.