Well, I realise that making it from scratch, especially using gates relaly is a hassle. Even though these chips are pretty cheap, you'll need a few of them, and, even though they don't require many uspport components (resistors, capacitors) they require lots of connections.
If you were dealing with non decimal counting, like minutes (go up to 60, rather than 100) then I probably would have abandoned the idea. But, there are chips specifically for counting 0-9, then reseting (CD4510, 74HC4510 etc..). Look here:
**broken link removed**
Alotof circuits there deal with 0-60, which you don't need. In fact, the chips can just be cascaded, one for each digit.
As always tohugh, things get tricky. With a 100Hz input, you could have 100ths, 10ths, seconds, and tens of seconds. The chip can count up or down, and has an output when it resets (when it over flows from 9, to 0, or 0 to 9). You'll need an accurate clock for this, preferably crystal. One last thing, thes chips have 'loadable' presets, meaning you can load in a preset value, allowing you to reset your counter to a specified time, but, for four digits, each one takes 4 lines, so thats 16 lines to pull high/low for the reset. Its starting to get complicated :/
I'm sure there is a chip which can do this all for you, although, as you found out, commercial products, and the chips that drive them, all seem to work in seconds and minutes, with no smaller resolution. I'll keep looking.
Blueteeth