I have been trying to figure out the best way to decode a tricky signal (A multi-level digital signal with random pulse widths) on 4 parallel lines.

The output should be a 4 bit digital code - I figure instead of decoding each signal with a separate ADC I can decode all 4 signals at once by just detecting signal transitions (represented by transient 1) The signal transitions could be as close as a few nanoseconds apart. I have attached a picture of the Signal in hopes of clarifying my problem. For on-chip processing what readout configuration could be fast enough ?. I'm thinking some type of monostable circuit - any feedback to point me in the right direction would be greatly appreciated.