Hi Roadrunner76,
I don't doubt that there may well be a few Olympic tales of those who only took up their discipline 4 years previous. But I'd have said that would apply in the vast majority of cases to Olympic sports that aren't truely based on sheer athletic performance.
For example, you might be able to become a superb fencer with only four years experience, or a superb amateur boxer too or an archer or shooter. There really are so many skills based disciplines where a dedicated individual might attain a world class level in four years of training.
But with running events, even in the cases of supreme talent, it is very rare to attain the types of fitness required of a peak athlete and I'd say the longer the distance of the running event, the less likely anyone might be to get to the level of fitness required within 4 years.
The best chance for running in my opinion would be for the many 15-16 year olds in the country who can run 11-12 sec 100m sprints just on their football training. We had one in our year. surely the UK must be full of them! Four years of sprint specific graft at that age with a decent coach might well be able to get some of those guys gown to 10 sec level.