Difference between MMAS-4 and MMAS-8

The difference between the MMAS-4 and the MMAS-8 is that the MMAS-4 has very low reliability (Cronbach's alpha is only .68) compared to the .83 for the MMAS-8.  The MMAS-8 also has been able to identify many more long-term outcomes compared with the MMAS-4.  
Specifically, there are many more independent variables such as beliefs in medication-taking behavior as well as strong associations with knowledge of the medical regimen and positive attitudes regarding medication taking behavior.  Long-term outcomes include a very strong correlation of the MMAS-8 with the proportion in control of their health condition, reduction in 30-day appointment-keeping, quality of life, morbidity and mortality.  

Journal Articles regarding determinants of nonadherence 
Retrospective Cohort Study of Medication Adherence and Risk for 30-day Hospital Readmission in a Medicare Cost Plan
Barriers to Determinants of Medication Adherence in Hypertension Management: Perspective of Cohort Study of Medication Adherence Among Older Adults
Predictive Validity of a Medication Adherence Measure in an Outpatient Setting
​Effect of Intensive Blood-Pressure Treatment on Patient-Reported Outcomes