Yes. Account 2 is not partied with a linked account of the SAME LEVEL, so he will only get 100% (base) XP. Accounts 3 and 4 will get 300% XP since they are linked and are the same level, even though a lvl 70 is in the group and boosting them.

If you wanted to boost 3 accounts with your paladin, you should have created account 2, then linked 2 to account 3, then linked 3 to 4. Then you could boost with the paladin and have all 3 extra accounts at 300% XP. I believe it's not too late to buy a 5th account and link 2 to 4, then 2 will be linked to 4, and they'll both get 300% XP.