Bandwidth and sampling rate are completely independent. The sampling rate must meet the Nyquist criteria to accurately reconstruct the waveform, but this has nothing to do with the bandwidth as defined by the -3dB amplitude response, which is why equivalent time sampling works.
As pointed out by tggzzz, a bandwidth limited signal can be reconstructed with a sampling rate greater than twice the bandwidth no matter where in the frequency spectrum it is, within the bandwidth of the sampler. The sampling function itself is equivalent to RF mixing, and the circuits can be identical. RF mixers make great microwave samplers when driven with a suitable pulse through their local oscillator port. The sampling part of an analog-to-digital converter can be modeled as a down-conversion mixer.
some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.
Is it true?
If you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and with a 3dB amplitude error.
A more useful oscilloscope specification is rise time, which in the general case in nanoseconds is 350 divided by the bandwidth in MHz, so 100 MHz yields 3.5 nanoseconds. Oscilloscope rise time needs to be several times faster than the rise time of the signal for an accurate display.