1.  Field of the Invention
The present invention relates to golf balls.  Specifically, the present invention relates to multi-piece golf balls including a core, an envelope layer, a mid layer and a cover. 
2.  Description of the Related Art
The greatest interest to golf players concerning golf balls is flight performance.  In particular, golf players place importance on a flight distance upon a shot with a driver.  By using a golf ball that has a long flight distance upon a shot with a driver, golf players can hit a second shot at a point close to the green.  A flight distance upon a shot with a driver correlates with a spin rate.  A golf ball with a low spin rate has excellent flight performance.  In addition, flight performance correlates with the resilience performance of a golf ball. 
Golf players also place importance on spin performance of golf balls.  If a backspin rate is high, the run is short.  By using a golf ball that has a high backspin rate, golf players can cause the golf ball to stop at a target point.  If a sidespin rate is high, the golf ball tends to curve.  By using a golf ball that has a high sidespin rate, golf players can intentionally cause the golf ball to curve.  A golf ball with excellent spin performance has excellent controllability.  In particular, advanced golf players place importance on controllability upon a shot with a short iron. 
Golf players also place importance on feel at impact of golf balls.  In general, golf players prefer soft feel at impact. 
Golf balls with a cover including a polyurethane are commercially available.  In general, polyurethanes are flexible.  Spin is easily given to this golf ball.  This cover contributes to the controllability.  On the other hand, if this golf ball is hit with a driver, this cover causes excessive spin.  This cover impairs the flight performance. 
Golf balls with a core and a thin cover formed from a polyurethane are commercially available.  This core includes a center and a mid layer.  The mid layer is formed from a hard synthetic resin.  This mid layer achieves an outer-hard/inner-soft structure of the core.  When this golf ball is hit with a driver, the core deforms significantly because the cover is thin.  The outer-hard/inner-soft structure of the core suppresses spin.  This core achieves a long flight distance upon a shot with a driver.  When this golf ball is hit with a short iron, a high spin rate is achieved by the flexible cover.  This cover contributes to the controllability upon a shot with a short iron.  This golf ball has hard feel at impact because the cover is thin and the mid layer is hard. 
There have been various proposals intended to improve various performance characteristics of golf balls.  U.S. Pat.  No.  5,816,937 (JPH9-248351A) discloses a golf ball including a core, an inner cover, a mid cover and an outer cover.  U.S. Pat.  No.  6,277,036 (JP2000-60998A) discloses a golf ball including a core, an envelope layer, a mid layer and a cover.  US2002/119840 (JP2002-191719A) discloses a golf ball including a core, an inner cover, a mid cover and an outer cover. 
In the golf ball disclosed in U.S. Pat.  No.  5,816,937, the mid cover is hard.  This golf ball has inferior feel at impact.  In the golf ball disclosed in U.S. Pat.  No.  6,277,036, the envelope layer is flexible.  This golf ball has excessive spin upon a shot with a driver.  In the golf ball disclosed in US2002/119840, the outer cover is hard.  This golf ball has insufficient spin upon a shot with a short iron. 
Golf players' requirements for golf balls have been escalated more than ever.  An objective of the present invention is to provide a golf ball having excellent flight performance, excellent controllability, and excellent feel at impact.