If you live in a very humid climate it may take longer for the oil to dry.
If the hide hasn't dried after 4 days you will need to do a couple of things.
First, use the rounded side of the Hide Scraper and scrape over the wet area. This is called staking the hide and basically helps the leather fibers to open up.
Next, you will need to pull the hide off of the board and hang it so that air can get to the hair side as well.
Once the oil is dry you will need to re-tack the hide to the board so you can use the sandpaper to finish the hide.