New project: Should I start with ExtJS 5 or 4?

Hi All,

I currently have a project running with ExtJS 3.4 and I know that moving to ExtJS 4 will be very painful so I know probably I will have to rewrite most of it.

Being so, should I start directly with ExtJS 5 even though it is in Beta in the hopes than sooner rather than later the final release will be done, or move to ExtJS 4 and later on have additional effort to migrate to ExtJS 5?

What's your opinion about it?