Try optimizing the catalog: http://lightroomkillertips.com/its-lightroom-optimize-your-catalog-day/
btw. the photos are not in the catalog. They are wherever you have imported them from/to.
The size you are quoting is the size of the lrcat file?
What is your OS and the exact version of LR?
My catalogue is larger than 4 GB, but there are only 358 photo's in it.
How can I reduce it?
If you are getting this error message, then the message itself is a mistake and you can ignore it.
Thanks for your answer.
Optimizing the catalogue has been done several times but without result.
Currently I am using CC 2017.0.1, but I had the problem before I upgraded from Lightroom 4 and Photoshop CS6.
I am aware of the fact that the photo's themselves are not in the catalogue, but all the manipulations are. (I keep track of the history)
I am using an iMac 27" late 2013, macOS Sierra version 10.12.3