So the client has a need you can fill. When an opportunity reaches stage 4,
it means you're working out what you'll do for that client.
It means further talking and another look at the probabilities. By the
time you're in stage 4 you're talking seriously to a client showing an interest
in what you can do. And the chance of you winning the project moves upwards
accordingly.
Of those 40 opportunities in stage 2, perhaps 16 will reach stage 4.
If your business involves consultancy or creativity - human value-add - stage
4 is where most of your energy should go; ideally the number of opportunities
in stage 4 matches the resources available for business development. (If you've
got 2 FTEs writing proposals, half of value propositions go forward to proposal
stage, and developing a proposal takes a week, you've got resource allocation
about right.)