Worth upgrading to Unity 4?

I too am in the same position, but also, I have had unity 4 installed as well for quite some time while continuing to use 3x for mobile builds. From what I gather, the only major noticable changes in my workflow are that the interface is slightly different, but with options to go to a similar one 3x used, in regards to the hierarchy stuff...

but also theres small code changes you might need to do, because of changed/obsolete odds and ends (atleast in C#) but mostly quick fixes. The builds for mobile I BELIEVE are using a newer more effecient building system (like clicking build from build menu to my android seems to go faster anyway) and maybe there is support for some features that wouldnt be possible in 3x but I have really not hit anything in 3x that stopped me in my tracks yet...

I'd say try it out...you can install it to a different directory by just changing the install path, and have 3x and 4x on your system, then you should definitely BACKUP YOUR PROJECTS before converting them to 4x projects - THERE IS NO GOING BACK!...well not real easily anyway...

ALSO like, I believe there is no longer support for real old devices (android devices without something unity needs now....maybe like < gingerbread) so perhaps if your game is based on something less intensive, and you plan to keep supporting real old phones, it might be best to stick with 3x... wish I could remember off hand what it was that changed, but if I do find it I'll update my answer with that information.

EDIT ok found it, its the ARM versions, 4x no longer supports older than 7 I am reading here: http://forum.unity3d.com/threads/73598-Before-you-buy-Android-for-Unity-ARMv6-and-what-you-should-know/page2