Soryu should be relatively close by Akagi and Kaga. Hiryu, on the other hand, steamed north for a considerable time so finding her may be difficult. Mikuma, was alone, if I remember correctly, so where she sank is another huge area. Soryu is the most likely to be found.
I understand from reports that there is very little left of Kaga. Yorktown appears to be in pretty good shape. I wonder if they'll ever find Hiryu, Soryu and Mikuma?