Intend to upgrade to LR-6 imminently.
Current LR-4 catalogue sits in applications on Desktop iMac, 1TB, 16GB Ram, updated to/running El Capitan OSX ver 10.11.4
Original LR-4 purchased & installed using Disc supplied. (liked the disc as simple to re-install if I had a problem/crash)
Do run Hrly Time machine back-ups of PC only to a separate Seagate hard drive>
(not the stand alone LaCie hard drives/duplicated, containing all my indexed* raw+Jpg + LR-processed Jpg photo's) in case of a crash.
Understand newer/current LR-6.5 version will read new XQD NEF raw files? & are now without disc, requiring direct online download ?
(* I do this for security/peace of mind, in case i ever lost the LR-4 catalogue for any reason)
What happens to my current LR-4 catalogue when i download LR-6.5 ??
Does it cleverly but simply assimilate my old LR-4 catalogue but with all the improved LR-6 functionality ?
Am i worrying about nothing, being too cautious again ?
When you will install Lightroom 6 and open your Lightroom catalog for the first time Lightroom will ask you to upgrade it.
Note latest version of 6.5.1.See Keeping Lightroom Up-to-Date
Select yes and you will have all images,edits in the new version.See How to create and manage catalogs in Photoshop Lightroom for more info.
So nothing will be lost and your Lighroom 4 catalog will be left intact after the upgrade.
Regarding the XQD card, If your operating system recognises XQD cards, Lightroom can read the data on them.
Hope that helps.
Actually nothing happens to your LR 4 catalog file. It will still be exactly the same as it was before you installed and opened LR 6/CC for the first time.
When opening LR 6/CC for the first time LR 6/CC will ask if you want to convert your present catalog to work with LR 6/CC. It will then make a Copy of your current LR 4 catalog, using a new file name, and then convert that copy to a LR 6/CC version.