I'm actually doing a number 4 this weekend. I'm going to use a 12x18" pan, and I cut out a 12x18" piece of cardboard (actually, it was probably a teeeeeny bit smaller than 12x18 because it fit IN the pan). Then I drew out my number 4, traced it with a sharpie so I could see the lines really well and I cut it out of the cardboard with an exacto knife. I just drew it free hand, but the templates that you found look like they would be helpful.
I'm baking the cake tonight, and because it's a large pan, I'll use a heating core. My PLAN is to put in enough batter in the pan so it overflows a little bit, that way when it's done, I can just level it with a knife when it's still in the pan. My cake board is about 12x18 as well, so I'll put the cake board on the pan upsidedown (touching my newly leveled surface), and I'll flip it over onto the cake board.
Then, I'll tort and fill the cake. I might have to do it in two pieces because it's so large (like cut it right down the middle). Then using the template I made for my number 4, I'll carve it out.
That's my plan anyway! I've never done one of these before but I keep going over and over it in my head and I think it'll work! HTH!