Upgrading Drupal module from v4 to v6

There is a lot of documentation dealing with converting modules from one version to another: http://drupal.org/update/modules

Interestingly enough, there is no documentation in going from 4.x to 5.x.  Having never dealt with that old version, I can't say what has changed.  I would expect a fair amount of customization is going to be required, as Drupal has changed significantly over time.

The first thing I would do is create a basic (essentially empty) module using the 6.x format.  Here's a link for some documentation to assist: http://drupal.org/node/231276

Once the framework is there so that the module is at least recognized by Drupal (in your dev environment, of course, not on the production server), you have two choices: copy all the code and fix as needed or copy functionality over one piece at a time.  Either approach is going to have some advantages and disadvantages.  Depending on the size of the module and your personal coding style, pick the most appropriate approach for your needs.

In general, I would suggest that copying functionality is the best approach, as it gives you the opportunity to make it a truly 6.x module instead of a hacked 4.x module running in 6.x.  You also have the chance to better leverage newer code and moudles.

Don't forget that you may be able to delegate some of the functionality to other modules such as CCK, if appropriate.  This allows the module to focus on the new functionality instead of re-inventing the wheel.