A 4-day test match finished today, in a quite unsatisfying draw. It was originally scheduled as a 5 day, but the entire first day was ruled out through rain, and so, according to law 13.3, the test was designated as a 4 day match, and the follow on target was reduced to 150 accordingly.
The reason it was unsatisfying was that another days play and this could have been an epic finish, rather than a tame one. It might have still been a draw. But it might have been a famous win for England, after their terrible first innings, or it might have been a morale boosting win for New Zealand.
Given that there is already a precedent of the match conditions being able to be changed in a result of a days play being abandoned due to weather, I think it would be sensible for there to be a reserve day tacked on at the end of a test, and if (and only if) an entire day's play is lost, that day is designated as a rest day, and the test is extended onto the reserve day.
There have been very few 5 day draws in tests in the last 3 years. Almost every draw has had 2 or 3 sessions lost to rain. Making an allowance for that might mean that we get a result in some of those matches.