There really is no science to it, I have no clue to this day as to why they show up and sometimes they don't. As in the most recent photo I posted for midday 8/20/16 , there is 2 ways I have been doing it.
Using the method mentioned above , on the left side is the older way i was doing it. you just list the last 3 digit winning number under it and add. using the method above depending on the winning pick 4 you place that under the method and add. The right side is based on the previous days winning midday or evening drawing. personally at the moment the older method is still doing better than the newer way.
As far as picking the digits is another beast, you will notice that most of the time the pick 3 winning number is beside the highlighted number in pick 4 list and pick 4 winning number is also in the pick 3. For the most part , If I see a 3 digit number off the highlighted digit I then look to see it in the pick 4 list, If it's there I write it down. I also do the same when looking for a pick 4 number to play in the pick 4 workout, If it's on both the pick 3 and pick 4 I play it.
An example would be is if I see 746 on the pick 3 one or more times coming off a highlighted number, I then look to see if it's on the pick 4 list . If it's there also off of a highlighted number like 467 I write it down in the order if I see it going left to right and up and down in that order for a straight play.
You will notice a lot of times both the winning pick 3 and pick 4 are right there together on both lists (old way). As for the new way , it's just testing right now but the older way is showing more. I play both Midday/evening draws, Gets expensive trying to play all the combinations and I don't recommend that unless you have a ton of money to loose that you don't care about. Try playing $1.00 box on a few before you play on straight, something is better than nothing.