Actually, the 4100 Classic and Plus (along with the 4120) use an entirely different back can than the 4020 and 4002 series. The 4100/4120’s back can is about 2x deeper than the 4020/4002 back cans, which is why, for example, a 4100U or 4100ES could be retrofitted directly into legacy 4100 back cans (as long as the door and dress panel are changed out). It’s rather silly how they did this, but I guess that’s how the engineers designing these panels wanted to do it back in the early 90s.