Let me guess: A flash/laser/whatever is started every 4 microseconds. Pulse starts when light is emitted, stops when it's reflection returns. You want to measure distance from 3.5 cm to 30.000 kilometers (with the 3.5cm precision)... Don't you by a chance work for some army? They are about the only ones who would ever aks for such precision and range.
If so, don't be a cheapo playing with '51 micros and go for a 10GHZ DSP like those in high-end gfx cards.


Alternative idea: If you can get your hands on a few simple gates fast enough, attach them in a row like below. Obviously some extra circuitry i.e. to drive the crystal or reset the circuit to zero, trigger readout when pulse ends etc will be necessary...

10GHZ   latch
--|[]|--|>-----+-[/2]-+-[/2]-+-[/2]-+-[/2]-+-[/2]-+-...
         |     |      |      |      |      |      |
        in     O0     O1     O2     O3     O4     O5..

IN is your pulse, negated, attached to the "latch" signal. O0...23 are bits of your output. This way you get a nice 24bit value in 0.1ns units, 0.2ns precision and value up to 16ms. (the 0th bit may be just as well 0 as 1). First few dividers and the latch must be of a very fast kind, once the frequency has been scaled down enough you can use slower parts.
...if parts working at such speeds are produced at all...