SMB2j is a 1-side disk game so you never flip the disk. The RAM adapter is too small to load the whole disk side though so it loads only 4 worlds at a time. This is why it pauses after beating stage 4-4, to load worlds 5 to 8.
Really? If that's the case then the game is going to lock-up on two more occasions on the N8Pro. Right after you beat 8-4 (to load the ending) and the other after you beat the game eight times and try to load worlds A-D (both have the exact same pauses)
*Edit ; Yep, sure enough. Attempted to load my eight-star SMB2j FDS file, and it froze when trying to access worlds A-D just as it does after world 4-4. Can I upload the modified ROM? (It's really just a template, all the levels have been removed and the flagpole is right at the beginning in most levels)