I've just finished reading Development Tools in 2006: any Room for a 4GL-style Language? - a 32 page PDF report based on an in-depth 4GL usage survey Jean Georges Perrin conducted earlier this year.
As well as providing insight into how people are currently using 4GL and variant languages, the report delivers a useful comparison of features between the main products (Aubit4GL, BDS, Genero, Informix I-4GL, Rational EGL, Hydra4GL) including pros, cons and recommendations for switching (or staying put).
Also illuminating are the features people would like to see in 4GL. Some might be expected like greater web development support, but a few surprises too.
The report is well researched and presented. If you have any interest in 4GL this study is definitely worth a look.