As announced a few days ago, a big Sakura Taisen announcement is going to take place on July 1st. The official fanclub pamphlet has however spilled the beans early and revealed the logo and subtitle of the next installment in the series: Sakura Taisen ~ Atsuki chishio ni ~ (Sakura Taisen: In the Hot Blood).
This could mean the "PS2 port(s) of the series" should have at least one new installment, despite ST4's "final" feel. The lack of a numeral, and the change in the typography/logo design however hints at a new direction for the series.
More details will be unveiled on the July 1st announcement, and we'll bring you the information as soon as it emerges, so make sure to check back RPGFan for more info.