Is a person considered sinful if he believes that one of the Four Imams or others erred in some of their ijtihadat (independent reasoning), whether based on other, more accurate ijtihadat currently available, or due to a belated realization?

Every mujtahid (independent legalreasoner) is sometimes correct and sometimes mistaken, except for the Messenger of Allah, peace and blessings be upon him. Scholars would engage in ijtihad, and some of them would find fault with others' conclusions. A scholar might even retract his own statement. It is not permissible to declare scholars mistaken or to delve into matters of khilaf (difference of opinion) and tarjih (preferring one opinion over others) except for a mujtahid scholar or a well-versed student of knowledge who has gathered evidence and made a preference based on proof. As for beginners and common people, they must ask and follow (taqlid). Knowledge is fiqh (deep understanding) and an innate ability; merely reading books is not sufficient. Caution is necessary, and one must not speak about religion without knowledge.