Well mr aheeq, I am sure you are aware that there are two types of medical study designs...case control and cohort. Cohort study is when you follow for the entire duration (4 yrs). But you can also do a retrospective study in one day if you want to and study the effect of four years of exposure by using a case control study. although the odds ratios obtained from thses studies are not as uselful as Relative Risks (RRs) obtained from cohort studies, it is still statistically signifant if the proper tests are done for data comparison and confidence interval determination. So IT IS POSSIBLE to study a four year effect in one day or less than an hour even! But you have to make sure that the sample size is proper and there are no confounding factors and biases that invalidate the study.
Please refresh medical statistics and then comment.
