The same question was asked back in the 70s too, after the birth of the One-day format. However, time has put every speculation to rest as both formats of the game continued to thrive under the same roof. But with T20 cricket being an instant success by throwing results in just about three hours of time, it seems, the alacrity of the people to watch rearguard action for over a five-day period has gone down considerably.

Earlier, since the inception of the Test cricket, five-day cricket was never the norm rather England and Australia, the pioneers of modern cricket, used to host three-day Tests with the last match of the series a timeless one if and only if the series was undecided by then. Due to the political instability in Pakistan in 1969-70, the touring England asked Pakistan to host four-day matches to finish off the series little early and five years later, India and West Indies had also played a six-day series decider at the Wankhede Stadium in Mumbai. However, barring the anomaly of the Super Test in 2005, every match in the format over the last 37 years has been scheduled to last for five days.

However, since the turn of the millennium, the percentage of Tests that go to the fifth day have been considerably decreased and 42.42 % of Tests have been finished in four or fewer days. In the 1990s, the percentage of Tests that ended within four days were 26.51% and so, there is a clear indication that the growing impact of T20 cricket upon the oldest format of the game. In 2017 alone, 11 out of the total 33 Tests have been finished within 300 overs, which is still lower than the newly proposed 400 overs. So, the four-day Test, which was first proposed in 2003 by Andrew Wildblood, then a senior international vice-president for International Management Group, seems like a good idea as while preserving the fundamentals of Test cricket, it would also lead to more exciting cricket because, after the advent of Twenty20, batsmen's inability to bide time has come to the fore. 

But, to make the idea a full-fledged and productive solution, a lot of things need to be addressed and nothing dramatic could ever happen overnight. According to Wildblood, each day of the Test match can be lengthened to 100 overs and by doing that it would mean that only 50 overs would be lost during the Test. For the record, of the 162 Tests played since the beginning of 2014, 135 matches have been ended with a result, and out of the 135, 112 of those finishing inside 400 overs. In that case, then every player and captain will bring a more attacking approach to the game and scoring rates will rise as well. Bowlers will be more attacking in their approach knowing that they have lesser number of overs to inflict an all-out on their opposition twice. Again, pitches used for Test matches, particularly in the sub-continent, are not suitable for five days anymore. With the wickets wearing away completely by the end of Day 4, they offer the spinners vicious amount of turn on day 5 which, on some occasions, makes it almost impossible for batsmen to surprise. A four-day Test would help to eradicate the pitch factor, which favours the team that wins the toss, and can help make the match more balanced. 

On the other hand, four-day Test matches could always start on a Thursday, and would, therefore end on Sunday evening and that will give a greater chance of a full stadium and better viewing figures than a Monday afternoon. England and Wales Cricket Board Chairman Colin Graves recently stated that playing without the fifth day "would save a hell of a lot of money from the ground's point of view and the broadcasters," also stating his view that games which go on to the fifth day often lose money if they finish early. Some people may argue that with the over rates are going down due to DRS, many Test might end on to be a tame draw and it is not a good idea to trim the Tests down to four days. But the point is an extra hour a day and efforts to push the run rates up can easily solve the problem. 

However, to be honest, all these sound well on pen and paper and executing them will take a lot of efforts for the sport’s stakeholders. But Test cricket is at crossroads and the experiments like – Day/Night matches and four-day Tests might act as a tool for restoring the viability of the format. A proper plan and natural radicalism by the administrators are important in this regard.