What is the algorithm used in A1 4.0 for determining values for priority and changefreq?

You can read how A1 software calculates importance of URLs:
http://www.microsystools.com/products/website-analyzer/help/website-analyzer-link-juice/

Some further information about priority values in XML sitemaps:
http://www.microsystools.com/products/sitemap-generator/help/xml-sitemap-priority-tag/

Incase you want to override importance and change frequency values:
http://www.microsystools.com/products/sitemap-generator/help/xml-sitemaps-creator-importance/


Hope this helps, otherwise let me know

Excellent - thanks for the quick and helpful reply!

By the way:


The Change frequency depends

last-modified
the revisit-after / revisit meta tag attribute
calculated page importance (used for priority)

To override Change frequency :
Switch of easy mode
Go through tabs: "Create sitemap" > "XML sitemap options"
Check "Override ChangeFreq".
Set dropdown to "*" value (as indicated in the tool tip, this removes the value)

For more help + screenshot, see:
http://www.microsystools.com/products/sitemap-generator/help/xml-sitemaps-generator-frequency/