Why are you using blending at all? Use a context filter to the last four months and have the domain of the filter be based on context.
There are 2 reasons why did I use blending:
1. To restrict the filter to last 4 months
2. Choose a particular date from a filter then all the bar charts should be plotted for last 4 months.
As an example: if I choose Aug-2018 then the bar should be plotted for last 4 months including selected month Aug-18, Jul-18, Jun-18 and May-18.