Importing from LV Server 3 was a doddle, less then 5 mins to do.
It works fine with our Ultravnc and LV3 clients installed on the computers on site. Tested out the new client and ultravnc but you can't use your old password, you can't even seem to change it. I looked into the configuration files and there is a field for a password, but changing that to what we used before didn't work.
But
What you have to do from what I saw if you want to install the new client, with the new ultravnc you need to go to your lv4 Server go to:
File > Options > VNC
In there is a little hyperlink next to text box labelled 'Reset', click that and it will put in the default password that is configured with the new Ultravnc, then it should start working again. You just gotta install it to all stations.
But again
Something else we discovered first time we tried the client without taking out the old ultravnc was that our lvserver could still connect to the computer using the old password, so it seems like it did not get overwritten by the exe.
Of course you can optionally choose to not install the ultravnc by going to 'Custom; on the installation
Hope this helps