I received this email from Adam Lawrence; it provided the information I needed. Essentially, the CLIENT API version works with later versions of the MySQL Service running on the Windows box. The php_info() details do not address the server version being used, and warnings about different versions between PHP and MySQL do not seem to be born out by other comments I received by email:
I doubt there will be any issues. I use php 4.3.2 with a MySQL 4.0.17
installation and its performance has been flawless.
What php is reporting is the compiled-in version of the MySQL client library
that it uses for database access. Generally, using a older client to access
a newer server shouldn't cause problems.
According to the php site, they intend to stop compiling-in the MySQL
library and will dynamically link to the libmysql.dll that comes with the
version of MySQL that you have installed. (i.e. 4.0.17, in your case).