How can a Muslim follow one of the four Imams without being a muqallid (blind follower)?

Followers of the schools of thought are not all on the same level. Some are mujtahids within the school who may differ with their imam if evidence becomes clear to them. This is what the great imams enjoined: to adopt the evidence and abandon their own sayings if they contradict the noble hadith. No one should blindly imitate a specific imam and never depart from his sayings. Rather, it is obligatory to embrace the truth wherever it is found. If the truth becomes clear to a Muslim, even if it contradicts his school of thought, he must follow the truth. As for Ibn Hazm, he was an imam and a mujtahid who forbade taqlid (blind imitation), and he was the imam of the Zahiri school.