Will High Frequency explode an LED?
High frequency will not explode an LED. it will simply turn it on and off very quickly. This is exactly what is happening in TV remote controls, albeit at 36kHz instead of 4MHz.

Normally, when you apply a frequency to an LED, when the signal goes high, the led turns on, and when the signal goes low, the LED turns off. Because of this, the perceived intensity corresponds to the duty cycle of the signal.

However,  if one half of the period is shorter than the rise or fall time of the diode, then it will not raise to full power when the signal is high, and you can no longer compute the perceived intensity just from the duty cycle - the perceived intensity will be less than expected.