If I enter 4 in my parameter value the filtered values coming as 3. that is one less in each case. What can be the possible reason here

There might be a problem with some other filters. For example, let’s say you have a filter using which you are controlling the top 4 and at the same time you are also using another filter which is finding the subset of the data. In such a case, the previous filter result can be changed a bit. Please check all your filters assigned to that worksheet and see if one filter is impacting The Other filter.