Checking prices, I see at most a $10 difference between a 4” and 5” MERV 16 filter. Also, the second filter will likely increase flow restriction and, counterintuitively, will cause you to need to replace both filters more often. If you can find a much cheaper 4” MERV 16 filter, you could probably make it work in place of the 5” filter with a couple of thin corrugated cardboard strips tensioning the thinner filter in place.

Do not install two filters in series. Why are you using MERV 16 filters?