Question: > 
> Many an opportunity is lost because a man is out looking for four-leaf
>  clovers.
> 
> 
> 

I have no idea what the quotation means. Is there any special meaning about four-leaf clovers that I may be unaware of?

Answer: 
My guess is: success comes more from conscientiousness (seizing opportunities) than from luck (four-leaf clovers). Or: don't rely on luck, take advantage of opportunities.
The only reference I can find right now says that the ratio of three-leaf to four-leaf clovers is ~10,000 to 1. I would interpret that saying as meaning that you're likely to miss real opportunities if you spend (waste) your time looking for a rare item to bring you good luck.