My guess is that Lightroom 6 converted the wrong Lightroom 4 catalog.
So, go back to Lightroom 4, find the catalog file that contains the proper 2015 photos, make a note of its exact folder location and file name.
Then, in Lightroom 6, use File->Open Catalog and point it to this exact folder location and file name found above.
Could you please let me know which operating system are you working on ?
Also is the Catalog showing completely empty ?
Or are there exclamation mark on the images in the Catalog ?
And please try to upgrade the Catalog once again to Lightroom 6.
Much appreciated found them.....
Sorted thanks for the help,
You are Welcome, Glad to help.