The giant scorpion is on the level 4 list for Summon Monster but the level 6 list for Summon Nature's Ally. This seems like an error. I know there are some other errors in the Summon lists as printed (and on the paizo srd). For instance, the riding dog appearing on the level 1 lists for both spells instead of just a regular dog.
I've seen the dog error corrected somewhere along with some others about ants and lizards, but I haven't been able to find anything about the giant scorpion. Based on the CR, HD, etc, of the scorpion relevant to the other stuff on the lists I'd guess that 4th level is the correct one. A lot of people feel uncomfortable changing stuff like this without some kind of official or semi-official word. If somebody could find (or provide) some I'd be thankful. I've looked around for a while but didn't see any useful info on this.