Namco Bandai made all of the core Summon Night games available on PSP and in Summon Night 4 you can see characters from every title in the series. Summon Night 4 has an extra chapter which is one giant crossover with characters from Summon Night 1, 2, and 3. Clear it and you can get them as extra units.
The extra chapter has new special attacks…
… and illustrations for event scenes made for the PSP version.
Summon Night 4 even has characters from Summon Night EX Thesis in it. Xiaomei’s shop connects players to the Infinite Corridor, a series of really difficult dungeons. You’ll run into members from the Midnight Sun here. If you manage to defeat them you can use them in battle.
That’s a fair amount of Summon Night fanservice in Summon Night 4. The PSP port comes out on November 15.