For the care of CML, imatinib is still used by most experts as first-line, and patients would then be "graduated" to dasatinib/nilotinib
if they are intolerant or show resistance to imatinib. So, sprycel is one of the best available drugs for CML. But, as to whether your dad is responding, that is for his managing hematologist
My guess is, at 4 years, he should have achieved some of the milestones of therapy, but he has to be continually monitored to ensure continued benefit since no drug can currently cure CML.