You should allow the upgrade of entire catalog to Lightroom 6 as it will retain all your editing done so far.
You can take backup of your Lightroom 4 catalog so as to maintain a backup for future reference.
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Allow the updating of the LR4 catalog.
You will have both available, although the LR4 catalog will not know about any work you do in LR6, and work you do in LR6 is not easily transferred back to LR 4. So the plan ought to be to do all future work in LR 6
Thanks for the reminder to backup. It took ages to convert to the new catalog but so far working OK. Seems there are some interface changes from LR4 I'm going to need to learn
Wanted to indicate this as also correct answer but seems it won't allow me to do that on both replies? I appreciate the speedy response and help and would like dj_paige to get credit as well
Thank you for reaching out on this. I have marked dj_paige's answer as helpful as it allows only one answer to be marked as correct.