Same size, different layout
As you can see, while the Raspberry Pi 4 Model B, on the left, is the same size as the Raspberry Pi 3 Model B+, it has a different layout.
The change has been necessary to incorporate the dual micro-HMDI ports on the board.
The new layout means side panels in cases for older Pi boards won't fit correctly. I also found the board overheated when playing video while using an older case, with the computer throttling performance to reduce temperature.