This is my usual go-to site for that. They have a ton of other options too (scroll down to the bottom of the page if you want to see those too!), such as dimension-based resizing, quality changed, file type conversions, etc, but the link will take you to the compression option.
If you click the drop-down below the image upload area, there should be a way to put in the max KB you want. I usually just put exactly 4000 and it works fine ^^