Introducing the GSTB400X-1A4, the newest model of the slim G-STEEL GSTB400 series. The GSTB400X-1A4 sports a lightweight yet tough carbon bezel, which is a combination of carbon fiber reinforced resin and carbon sheets. This stylishly stealthy timepiece is styled with a black resin band and a black IP case with red accents. The GSTB400 is made possible by even further miniaturization of its parts and thinning of its modules without sacrificing any Bluetooth or solar function. Power consumption has also been significantly reduced. In addition, the carbon core guard structure of this model helps to make it the thinnest of the G-STEEL Series (as of Apr 2021) while maintaining a high level of impact resistance. The thickness from the back cover to the bezel top is only 12.9 mm, which is more than one millimeter less than the GSTB100 model. In terms of design, the number of button guards has been reduced as much as possible, enabling the use of large stainless steel buttons that are slim yet easy to operate.