>>531634690
You want to farm 3-2 for Sōryū and Hiryū. They can drop from mob nodes and there are two of them. 3-4 has Yorktown dropping in any battle but Akagi and Kaga are boss node drops only.

3-2 has better gear blueprint drops than 3-1 or 3-3 (which have the same ship drops), and even better than 4-1 imo.