Hi,
In theory, you can set the radio on for 1 second in order to send 1Mbit; however, that is not possible because ANT is the only one that controls and manages when the radio is on and off. Therefore, whether or not ANT module can be configured to be on for 1 second and extending the ON time to 5 seconds, the answer is no you can't again because it is only controlled by ANT.
The reason for not setting the radio on for the entire 1 second, is because the power consumption will be high resulting the batteries to burn!
As for sending simultaneously at 4 channels, it is not possible to burst two channels at the same time because the radio will be occupied by the burst of one channel only. Also, the rate 20kbps is the total rate no matter how many channels are open. Therefore, whether one channel or 4 channels are open the total rate will always be 20kbps.