The center frequency of channel 4 in the 2.4 GHz band is 2.427 GHz (2427 MHz). The center frequency of a channel is the midpoint of its frequency range, where the signal strength is highest and most concentrated. The center frequency of channel 1 in the 2.4 GHz band is 2.412 GHz (2412 MHz), as given in the question. The center frequency of each subsequent channel is obtained by adding 5 MHz to the previous channel's center frequency, since the channels are spaced 5 MHz apart from each other in this band. Therefore, to find the center frequency of channel 4, we need to add 15 MHz (5 MHz x 3) to the center frequency of channel 1:

2.412 GHz + 0.015 GHz =2.427 GHz

Alternatively, we can use a formula to calculate the center frequency of any channel in the 2.4 GHz band:

Center frequency (GHz) =2.407 + (0.005 x Channel number)

Using this formula for channel 4, we get:

Center frequency (GHz) =2.407 + (0.005 x 4)

Center frequency (GHz) =2.407 + 0.02

Center frequency (GHz) =2.427Reference:1, Chapter 3, page 85;2, Section 3.2