Pros

Nicer interface from version 3

Cons

- Upgrade (version 3 to 4) does not work out of the box
- Requires lots of configuration to get working properly
- Have never been able to get Nginx to work
- No support - haven't gotten responses to email
- Licensing issues - does not accept license key from Mamp Pro 3, so would not do this as an upgrade. might work if you uninstall version 3 and purchase Mamp Pro 4 full version and not as an upgrade.
- Big footprint - over one gigabyte of disk space for installation

Summary

Mamp Pro 3 worked great for years, and I was generally happy with it. As my needs become more advanced, I started using virtual machines to mimic the exact same environment as the production server, so there were no surprises after deployment. Then I was mainly use Mamp for the ability to set something up quickly as a temporary measure to test out a theme or plugin.

Once Mamp Pro 4 was released, I was prompted for an upgrade every time I started Mamp in the usual obtrusive popup, despite checking the box "Skip this upgrade." I finally decided to do the upgrade and didn't think it would hurt anything. To my regret, the upgrade broke all sites I currently had set up. I got permissions (403 forbidden), database errors "Error establishing database connection" and all sort of other problems.

Support for this product is almost nonexistent. I was able to get an email response initially to get the licensing issue partially resolved. But I have not gotten any responses from subsequent emails regarding tech support, and further licensing issues.

I wouldn't waist my time or money with Mamp Pro 4. If you don't want to start using virtual machines (Vargrant with Virtualbox for example), there're numerous other options for full stack tools like Mamp (paid and free) with good support, including Desktop Server, AMPPS, and XAMPP to name a few.