The proper system is to compensate date #2/date #4. That way it is measurable what the Shadchan has provided (a quality idea), and shadchanim are treated as succeses for prodcucing that which is in their hands.
This method is by far the best way to
1. Encourage all the people (especially young couples) who dabble
in shidduchim to stay the course and not quit.
2. To encourage those already involved in shidduchim to focus on the slightly older girls.
It has been tried tested and is the process of being expanded.