You can read the details over at Lightroom Journal. This is a relatively minor update compared to those in the past, but it could prove significant for people on Leopard who have been experiencing problems printing to some Epson models: 

• “Compatibility with legacy printer drivers on OS X 10.5 (Leopard) has been updated. I’d like to thank everyone who commented on my earlier postregarding this topic.  Some valuable feedback was provided and by working directly with Apple and Epson we are able to provide a resolution through this Lightroom update.”

One new issue that seems to be popping up in 1.4 is related to DNG conversion on XP. Converted DNGs taken into Develop will display “An unknown error occurred” even though the image can still be adjusted. I believe the issue is being investigated, so you may want to hold off on converting to DNG on XP for the time being.