There is no 1 right answer to this as far getting to a jpg. Yes, the file should be a jpg for posting and in sRGB color space, IMO.
You can use LR, save for web, which will convert the color space and save the image as jpg, you can pick the size. The 4MB limit is per image, Save it as 72 dpi, for web viewing, no larger.
If you don't have LR, then there are many actions, you can pick up both free and for a small charge that will do this:
In CC,
You need to do resample and downsample the image for the web. You have a 20MB tiff. I would just go to image size, and convert the file there. Convert the dpi to 72 and make sure the image is no larger than say 11 x 16 inches. Down sample it and then convert the color space to sRGB if you are not there already. Save it as a jpg, quality 9 to 11 and look for a final size of between 600KB to 1.2KB or .6MB to 1.2MB. That should be plenty large enough.
I am sure everyone has their own way of getting here.
This is just one idea. My my own work, I have a quick action I run in CC that does all of above in a simple 1 step process.
Paul