Step 4 is the step after all body work is done. If it's the side of a car for example, all the body work should be completed, pretending there isn't even a door gap there. All the body lines should be finished and straight with body work. The body filler should be in 80 grit. Basically, it should be completely ready for primer, other than the door gap.
More than likely, there is some body filler bridged over the gap. This is completely ok, and actually recommended. Block sanding across panels is easier when there isn't a gap there. You don't want the gap actually filled, just bridged over the gap; there is a big difference. Please note, the edge of the quarter panel can have body filler on it. This is the reality of taking these projects to this level with body work. The edge of the door should be metal.
Now it's time to cut the gaps open! This might seem like a daunting task if you've never done it before. You will only need a thin cut off wheel, and a couple minutes of patience. Here are the key points of cutting the gap open.
- GO SLOW!! This is very important! Just grabbing cut off wheel and running it wide open will be a disaster. It should be running barely fast enough that it stays spinning as its cutting through the filler.
- There is almost always somewhere to start, where you can see where that panels start and stop; like the bottom or top of the door. Start there.
- Make your best guess to the direction the gap is going, and start cutting. The goal is to not hit metal, but, you will. Use the metal as a guide, it just means you hit one of the panels, and need to move the cut off wheel over. So, adjust and keep going. If you slice into the metal, you were running the cut off wheel too fast. It will not cut the metal at all if you're using it at the right speed. Go slow!
- Once you make the cut all around the door, you will probably need to sand the filler back on the hinge side, so there is clearance for the panel to open.
- The attached video shows grinding the filler back in the door jam. There will be extra filler the needs to be ground away, and taper the edge of the gap back until there is no voids in the filler, and its adhered nicely to the metal. Usually there are some voids right at the edges of the gap.
- After the filler is tapered back with a 3" grinder, us some 80 grit sand paper to smooth out the filler and get rid of the grinder marks.
- All of the filler must be cleaned out of the gaps, the metal must be clean, and properly prepared for more body work.
The picture above shows what the filler will normally look like after the door gaps are cut back open. Don't worry about trying to sand it straight or anything, just clean it up back to bare metal in the gap. 
Above shows using a grinder (slowly) to clean the filler up. This is also putting a taper in the edge of the filler.