I have a JPG file in Lightroom CC (latest version) and I want to export it reducing the file size from 4mb to 2mb. No matter what I change it comes back telling me to increase file size. How can I reduce it before I export?
It isn't possible to reduce the size of the file before it is exported because Lightroom does not modify the master file. It would be necessary for you to reduce the size, in dimensions, sufficiently as well as reduce the quality sufficiently to constrain the file size to meet your requirements. And that would be controlled in the export settings when you create the new copy.
In addition to JimHess' suggestions you can help make the Export file size smaller by insuring it isn't over-sharpened or has noise. Use the Detail panel controls at 100% view and adust the image so that there is no visible noise, which will inflate the export file size.
Some 4GB Jpg files will and some won't reduce to less than 200kb in LrC. It depends on the image. If it a blue sky it will but if the image has a lot of detail it will not. That is just the nature of Jpg compression.
One other thing to keep in mind is the file size. By this I mean the pixel dimentions. If you reduce an image by 50% (e.g., 4000 pixels wide down to 2000 pixels wide), you will decrease the storage size of the image by 75%
So be sure you find out what the actual size of the image needs to be before you go crazy trying to cut into the compression too much.
reducing the file size from 4mb to 2mb.
That would be 2000Kb (not 200Kb- or is this a mis-print in your subject header?)