The 6000 series replaced the 5100 series. The 6100 was the regular frame, and the 6600 was the double frame. While they didn't put it on the decal, the number of rows would be the last 2 digits. So a 4 row double frame would be a 6604, 6 row a 6606, and so on. The forward folding planters were the 6180, with only the last digit indicating row number. 6182 was 12 row, 6186 was 16 row.

I don't remember a having 4 row, but we had a 6 row that is the one I now use on the farm. The double frame is the way to go, lots more room for attachments, and rocks squeezing between the rows don't break off the gauge wheels on the row units.