I have been a database dilettante for the past nine years. I have developed some projects for clients using Filemaker Pro and am starting to use PHP and MySQL exclusively for my database needs.
Throughout this period, I have been aware of 4D and have even played around with a demo of it (albeit years ago), but have never really noticed that it had a lot of traction in the database world. This could surely be as a result of me simply not being properly exposed to the database market, but I'd like to think that as a proper engineer, I would stay abreast of such things

. My impression is that 4D simply isn't very common. It's certainly way behind PHP/MySQL in this regard.
I would stick with PHP/MySQL. It's free, and you certainly can't beat that. You also mention that you've stayed away from 4D because of its learning curve. Sure, it's GUI-based, but you can get some sweet front ends for MySQL that make it easier to visualize how your database is working. There is of course phpMyAdmin (
www.phpmyadmin.net), a must-have for MySQL users (it will even help you by generating PHP code) and there are some interesting commercial/shareware applications that provide nice front ends as well (
http://www.macosguru.com/macosguru3/products/ for instance...I tried a demo and liked it a lot!).
(There's always OS X Server...)
Ultimately, I think the choice comes down to convenience, function and price. What do you need your server to do? How soon do you need to have a solution in place? What sort of financial burden can you handle?
In my opinion, the only advantage Webstar has is speed of deployment and ease of configuration....
Tell us more about what you want to do. There is no one-size-fits-all solution out there.
-Peter