4010 and 4020 are virtually the same in appearance. Same clutch housing and all bolt patterns etc. are the same. 4020 tractor as a whole is on average 1000 lbs heavier. All castings and trans gears are slightly heavier, as well as the final drives. Ring and Pinion is a different ratio, as are the finals. 4020 R/P + 4010 finals = 18% faster than stock. 4010 R/P + 4020 finals = 13% slower than stock (I think!). If you are building light, you will want the 4010 for sure. Also, they have (4010) light & heavy axle housings as well, but I can't remember the casting #. I guess it depends on what you are planning to do with it. Hope this helps!!
just as a note 4020 transmission case is longer than the 4010 so the 4020 transmission parts will not fit in a 4010 if you were planning on using the heavier gears in the 4010
