5x5 or 4x4 after matching penalties

[OnDeck]

I am a baseball umpire and a LONG time follower of college hockey...and I have ALWAYS been confused by this: when both teams receive minor penalties on the same play, what determines if play restarts "5 on 5" or "4 on 4"? I used to think it had to do with whether the infractions occurred during play or after the whistle...or that it was different when infractions were simultaneous as opposed to one being retaliation...but after seeing several hundred games over the years, I've never been able to figure this out.