The ruling is bizarre and indicates the jury did exactly what they are not suppose to do - compromise. There is NO rationale by which he is guilty of attempted murder on 3 not killed, but in disagreement over the one he did. It means the jury decided they just wanted to go home and agreed on a middle ground. Either he meant to kill someone or not. While some will be glad he was convicted on anything, it is bizarre.
I also think it would poise a double jeopardy to retry him on one charge.
This also is an example of a lazy defense attorney. The defense should have insisted on a separate trial for each charge - which the defense can do and didn't. Now the prosecutor can take the 4 hung-jury and present it as fact that he attempted to kill the other three since there is a jury finding to that effect.