My advice, is to finish building the LR3, or build the LR3 if you have a 3D printer (printing the parts yourself).
If the LR4 comes out fast you can use it to make some parts for the LR4 and make life a lot easier, cheaper. You will get to learn some things and nearly everything gets used again from the hardware kit. The upgrade is a modest one. On top of that you can abuse the heck out of a new machine learning things, both in use and in assembly. Don’t wait to learn all the skills transfer over.
If the LR4 takes a while to come out then you will be a pro by the time it does. All the rest holds true.
Just yesterday evening, a 4mm change in CAD made a vast majority of the parts change. That is with only one fully built LR4 in the wild. It can get worse or we happen to catch everything on the second try.