I would like to build fast peak detector for 10ns pulses, PPM, 250ns repetition (4MHz), amplitude from 10mV to 5V.
PPM? Pulse position modulation?
What exactly is it that you want to detect if the repetition rate is 4MHz? Do you want every pulse amplitude?
Is there a strobe signal available?
If a strobe is available, then there are some sampling ADCs which can do this directly.
If no strobe is available, then 10 nanoseconds should be possible with an open loop diode half-bridge based peak detection circuit. Figure 21 on page 15 of Linear Technology application note 61 gives some idea of how this works. A faster buffer, faster transistors, and lower hold capacitance would be needed.
If you are looking for peak detection like modern DSO use, then they just sample the input at the full sample rate and keeps the highest values within a given duration. CCD based DSOs did it some other way in the analog domain but I am not sure how.